ASoC: soc-cache: Add reg_size as a member to snd_soc_codec

Simplify the use of reg_size, by calculating it once and storing it in
the codec structure for later reference.  The value of reg_size is
reg_cache_size * reg_word_size.

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