ASoC: wcd9xxx: Fix RX Ear register gain values

Rx Ear Gain register on WCD9306 supports different gain
values from 6DB to -12DB. It was wrongly configured with
2DB. This change fixes the issue and adds support for
correct gain values for Ear PA Gain.

CRs-Fixed: 520688
Change-Id: I6493cf63a1d36d9c9c0a3086b645b92f8c3de588
Signed-off-by: Venkat Sudhir <vsudhir@codeaurora.org>