ASoC: wcd938x: add support to retrieve codec variant

Machine driver requires a mechanism to know the codec variant
to limit supported configurations respective to that codec.
Add a function to return the wcd938x codec variant.

CRs-Fixed: 2488322
Change-Id: I16631cccb3abd32c933389e110e5b5a6b762a0e2
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>