ASoC: wcd934x: add null check for soundwire control data

Soundwire control data is NULL if there is no soundwire slave
device enumerated.
Add null check to avoid panic.

Change-Id: Ief60d69c36c2a9831825f38da2c9a3f6dd5f13dc
CRs-Fixed: 2020293
Signed-off-by: Meng Wang <mwang@codeaurora.org>