rtc: qpnp-rtc: Add NULL pointer checks

In shutdown hook, null pointer check is missing.
So during probe, if RTC H/w is not enabled, rtc
driver data will not be initialized. This patch
adds null pointer checks in shutdown hook.

Change-Id: I1ecdaef8415394c853ce9658ec0da0946f08ad92
CRs-Fixed: 545637
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>