mfd: Fix WM8994 IRQ register cache restore on resume

When the byte swap was factored out into the per-register I/O functions
the register restore for the IRQ mask cache (which we use and store in
CPU native format for the interrupt handler) was not updated to do a byte
swap when it uses the bulk I/O. Fix this by writing the cache out one
register at a time.

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