drivers/rtc/rtc-mrst.c: remove redundant dev_set_drvdata

Driver core sets the driver data to NULL upon device_release or on probe
failure.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>