rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ

It is redundant to do irqsave and irqrestore in hardIRQ context, where
it has been in a irq-disabled context.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1612355981-6764-2-git-send-email-tanxiaofei@huawei.com