drivers/rtc/rtc-tegra.c: properly initialize spinlock

Using __SPIN_LOCK_UNLOCKED for a dynamically allocated lock is wrong and
breaks the build with PREEMPT_RT_FULL.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Andrew Chew <achew@nvidia.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>