drivers: mfd: Mark ANC coefficients registers as volatile

Mark ANC (Active Noise Cancellation) filter coefficients
as volatile because codec hardware can auto update the
registers based on coefficient pointer register update.

Change-Id: I65aef7d7322db0ab3815d69e19cb97c7a2e044b2
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>