ASoC: wcd9xxx: check NULL pointer access for wcd9xxx

It may be possible that device up gets called
before slim probe and this leads to NULL pointer
access. Avoid the NULL pointer access before
processing device up.

Change-Id: Ie698b1b60099afdb26b9e723916630f9b2b97221
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>