rtc: Suppress duplicate enable/disable of WM8350 update interrupt

Unlike the wm8350-custom code genirq nests enable and disable calls
so we can't just unconditionally mask or unmask the interrupt,
we need to remember the state we set and only mask or unmask when
there is a real change.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: rtc-linux@googlegroups.com
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>