drivers/rtc: remove IRQF_DISABLED

Since commit e58aa3d2 ("genirq: run irq handlers with interrupts
disabled") we run all interrupt handlers with interrupts disabled and we
even check and yell when an interrupt handler returns with interrupts
enabled - see commit b738a50a ("genirq: warn when handler enables
interrupts").

So now this flag is a NOOP and can be removed.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>