ASoC: wcd9310: Return on an invalid switch value

In tabla_get_mbhc_micbias_regs the switch statement for the calibration
bias should never get to the default clause, but if it does return out
of the function and avoid a possible compiler warning.

Change-Id: Ic0dedbadf889a8e4ef9de0a906da0f97bc0f6a6d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>