rtc: ds1374, fix lock imbalance

When i2c_smbus_read_byte_data fails in ds1374_work, we forgot to unlock
the held lock.  Fix that.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>