mfd: Push byte swap out of WM8994 bulk I/O

For bulk I/O it is both convenient and more sensible to pre-swap the data
rather than doing the swap as part of the I/O operation so move the byte
swaps we're currently doing into the core write function into the register
based functions, giving the bulk write function a straight pass through
to the chip.

This leaves reads inconsistent, this will be addressed as a followup patch.

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