ASoc: wm8731: add 32bit mode.

This patch adds 32 bit word capability to the wm8731 driver. The wm8731
codec is capable of handling 32 bit word sizes, however that has not
previously been activated in the codec driver.

Signed-off-by: Matt Flax <flatmax@flatmax.org>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>