mfd: Implement tps65910 IRQ cleanup

The tps65910_irq_exit() cleanup function was generating a warning from
sparse due to the lack of a prototype. This wasn't causing GCC warnings
as the driver wasn't cleaning up its IRQs on exit at all so there was no
use of an unprototyped function.

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