posix-clock: fix up after realtime merge

So 562c33e0 "posix-clock: posix-clock: Fix unbalanced locking in
pc_clock_settime()" moved around timespec64_valid_strict() call to fix
an error path, but after de54723b time: Change posix clocks ops
interfaces to use timespec64 we did not have that problem any more.

So revert rest of the patch, and things should be fine.

Signed-off-by: Pavel Machek <pavel@denx.de>