ASoC: wcd9xxx: Set HPH PA register to volatile

Set HPH PA register to volatile to make sure the register will be
updated with proper bits during bringup.

CRs-Fixed: 625324
Change-Id: I8334a66644dca536f0bcfef562b7f64372a47d32
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>