ASoC: WM8776: Removed unneeded struct member

The member reg_cache is not used at all and therefore it should be
removed.  This member was usually needed for older versions of ASoC
that did not handle caching automatically and had to be done in the
driver itself.

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>