mfd: Remove copy from WM831x I2C write function

This saves us allocating an array on the stack, giving a meaningless
performance improvement and ensuring that if drivers ever do large writes
we'll not allocate large arrays on the stack.

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