rtc: mc146818: Reduce spinlock section in mc146818_set_time()

No need to hold the lock and disable interrupts for doing math.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220541.709243630@linutronix.de