drivers/rtc/rtc-twl.c: fix threaded IRQ to use IRQF_ONESHOT

Requesting a threaded interrupt without a primary handler and without
IRQF_ONESHOT is dangerous, and after commit 1c6c6952

 ("genirq: Reject
bogus threaded irq requests"), these requests are rejected.  This causes
->probe() to fail, and the RTC driver not to be availble.

To fix, add IRQF_ONESHOT to the IRQ flags.

Tested on OMAP3730/OveroSTORM and OMAP4430/Panda board using rtcwake to
wake from system suspend multiple times.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>