asoc: wcd938x/wcd937x: add null pointer check for mbhc variable

Whenever MBHC is disabled through kernel config file,
particular vaiable of mbhc is not guarded for NULL
pointer check causing kernel panic,
Hence add NULL check for that pointer variable.

Change-Id: Iae4950ceb076239a130b4f033cb2f08cd65a92a9
Signed-off-by: Shalini Manjunatha <quic_c_shalma@quicinc.com>