ASoC: wm8974: set cache type for regmap

Attempting to use this codec driver triggers a BUG() in regcache_sync()
since no cache type is set.  The register map of this device is fairly
small and has few holes so a flat cache is suitable.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org