ASoC: wm8994: Fix missing break in switch

Add missing break statement in order to prevent the code from falling
through to the default case.

Addresses-Coverity-ID: 115050 ("Missing break in switch")
Reported-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org