mailbox: imx: fix crash in resume on i.mx8ulp

mailbox: imx: fix crash in resume on i.mx8ulp

BugLink: https://bugs.launchpad.net/bugs/1969110
[ Upstream commit 8219efd08a0aa1d7944bdb66d84ba57549258968 ]

check 'priv->clk' before 'imx_mu_read()' otherwise crash happens on
i.mx8ulp, since clock not enabled.

Fixes: 4f0b776ef5831 ("mailbox: imx-mailbox: support i.MX8ULP MU")
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 8bddc0227e78edea1da2da357d1e184ccd2c25e8)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>