mfd: Convert WM831x interrupt masking to enable/disable operations

The WM831x interrupt masking support is a much better match for the genirq
enable and disable operations than for the mask and unmask operations. The
latter are intended to used during interrupt handling for temporary changes
which isn't really practical on a slow bus. Convert the operations over to
match this.

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