rtc: ds1343: remove unnecessary mutex

Use rtc_lock and rtc_unlock to lock the rtc from the interrupt handler.
This removes the need for a driver specific lock.

Link: https://lore.kernel.org/r/20191019204941.6203-7-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>