mfd: tps65912: Declare and use tps65912_irq_exit()

Clean up interrupts on exit, silencing a sparse warning caused by
tps65912_irq_exit() being defined but not prototyped as we go.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>