UBUNTU: SAUCE: ptp: free ptp clock properly

UBUNTU: SAUCE: ptp: free ptp clock properly

There is a bug in ptp_clock_unregister() where pps_unregister_source()
can free up resources needed by posix_clock_unregister() to properly
destroy a related sysfs device.

Fix this by calling pps_unregister_source() in ptp_clock_release().

See also:
commit 75718584cb3c ("ptp: free ptp device pin descriptors properly").

BugLink: https://bugs.launchpad.net/bugs/1864754
Fixes: a33121e5487b ("ptp: fix the race between the release of ptp_clock and cdev")
Tested-by: Piotr Morgwai Kotarbiński <foss@morgwai.pl>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>