ASoC: wcd9306: Avoid codec read/writes if device is down

When SLIMbus is down, read and write codec registers over
SLIMbus cannot be performed. Set codec device down whenever
SLIMbus device down notification is received so that read and
writes can be avoided safely. Later when SLIMbus device up
notification is received, codec device down state will be
cleared and read/writes can be sent as usual.

Change-Id: I6ab20e272aee4e8f9a7f5319f34527b32923245f
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>