ASoC: WCD9310: Include the MBHC mic bias registers in private data

The microphone bias that MBHC uses will be board specific and will
not change once it is decided for the board. Add common function to
compute the commonly used mbhc mic bias registers and save them in
private data so that these do not have to be computed each time.

Change-Id: Ibd0f1029cd8ca663d32e3e2eac0cc76c1e110abe
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>