ASoC: Don't squash 16x8 registers down to 8 bits

Currently we'll force all registers to fit in 8 bits before passing
down to the I/O function. Looks like a cut'n'paste bug.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>