rtc: snvs: Fix usage of snvs_rtc_enable

rtc: snvs: Fix usage of snvs_rtc_enable

BugLink: http://bugs.launchpad.net/bugs/1783418
[ Upstream commit 1485991c024603b2fb4ae77beb7a0d741128a48e ]

commit 179a502f8c46 ("rtc: snvs: add Freescale rtc-snvs driver") introduces
the SNVS RTC driver with a function snvs_rtc_enable().

snvs_rtc_enable() can return an error on the enable path however this
driver does not currently trap that failure on the probe() path and
consequently if enabling the RTC fails we encounter a later error spinning
forever in rtc_write_sync_lp().

[   36.093481] [<c010d630>] (__irq_svc) from [<c0c2e9ec>] (_raw_spin_unlock_irqrestore+0x34/0x44)
[   36.102122] [<c0c2e9ec>] (_raw_spin_unlock_irqrestore) from [<c072e32c>] (regmap_read+0x4c/0x5c)
[   36.110938] [<c072e32c>] (regmap_read) from [<c085d0f4>] (rtc_write_sync_lp+0x6c/0x98)
[   36.118881] [<c085d0f4>] (rtc_write_sync_lp) from [<c085d160>] (snvs_rtc_alarm_irq_enable+0x40/0x4c)
[   36.128041] [<c085d160>] (snvs_rtc_alarm_irq_enable) from [<c08567b4>] (rtc_timer_do_work+0xd8/0x1a8)
[   36.137291] [<c08567b4>] (rtc_timer_do_work) from [<c01441b8>] (process_one_work+0x28c/0x76c)
[   36.145840] [<c01441b8>] (process_one_work) from [<c01446cc>] (worker_thread+0x34/0x58c)
[   36.153961] [<c01446cc>] (worker_thread) from [<c014aee4>] (kthread+0x138/0x150)
[   36.161388] [<c014aee4>] (kthread) from [<c0107e14>] (ret_from_fork+0x14/0x20)
[   36.168635] rcu_sched kthread starved for 2602 jiffies! g496 c495 f0x2 RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0
[   36.178564] rcu_sched       R  running task        0     8      2 0x00000000
[   36.185664] [<c0c288b0>] (__schedule) from [<c0c29134>] (schedule+0x3c/0xa0)
[   36.192739] [<c0c29134>] (schedule) from [<c0c2db80>] (schedule_timeout+0x78/0x4e0)
[   36.200422] [<c0c2db80>] (schedule_timeout) from [<c01a7ab0>] (rcu_gp_kthread+0x648/0x1864)
[   36.208800] [<c01a7ab0>] (rcu_gp_kthread) from [<c014aee4>] (kthread+0x138/0x150)
[   36.216309] [<c014aee4>] (kthread) from [<c0107e14>] (ret_from_fork+0x14/0x20)

This patch fixes by parsing the result of rtc_write_sync_lp() and
propagating both in the probe and elsewhere. If the RTC doesn't start we
don't proceed loading the driver and don't get into this loop mess later
on.

Fixes: 179a502f8c46 ("rtc: snvs: add Freescale rtc-snvs driver")
Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>