ASoC: soc-cache: Factor-out the hw_write() specific code

The handling of all snd_soc_x_y_write() functions is similar.
Factor it out into a separate function and update all functions
to use it.

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