ASoC: wcd_cpe_core: Mark functions as static

Some of the functions that are supposed to be static are not declared to
be static. Cleanup the driver to make sure all functions specific to
this file are marked as static.

Change-Id: Ie2e0a7a1df5d49e8b6ed2af90b1a727e5c08746d
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>