drivers/rtc/rtc-twl.c: ensure all interrupts are disabled during probe

On some platforms, bootloaders are known to do some interesting RTC
programming.  Without going into the obscurities as to why this may be
the case, suffice it to say the the driver should not make any
assumptions about the state of the RTC when the driver loads.  In
particular, the driver probe should be sure that all interrupts are
disabled until otherwise programmed.

This was discovered when finding bursty I2C traffic every second on
Overo platforms.  This I2C overhead was keeping the SoC from hitting
deep power states.  The cause was found to be the RTC firing every
second on the I2C-connected TWL PMIC.

Special thanks to Felipe Balbi for suggesting to look for a rogue driver
as the source of the I2C traffic rather than the I2C driver itself.

Special thanks to Steve Sakoman for helping track down the source of the
continuous RTC interrups on the Overo boards.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Tested-by: Steve Sakoman <steve@sakoman.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Tested-by: Shubhrajyoti Datta <omaplinuxkernel@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>