ASoC: wcd-mbhc: Fix null pointer dereference for mbhc

IRQ data pointer is corrupted due to slimbus NGD errors.
This may happen as IRQ will be received from slimbus.
Check for mbhc null pointer before dereferencing to avoid crash.

Change-Id: I142ee535c6d643b53063b57c0f45414aff8b6304
Signed-off-by: Vatsal Bucha <vbucha@codeaurora.org>