staging: Android: Fix NULL pointer related warning in alarm-dev.c file

Fixes the following sparse warning:
drivers/staging/android/alarm-dev.c:259:35: warning: Using plain integer as NULL pointer

Cc: Brian Swetland <swetland@google.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>