ASoC: codecs: Fix mbhc data NULL pointer dereferencing

Initialize completion structure in wcd_mbhc_initialize
and wcd_mbhc_init to handle irq handlers getting called
before the completion structure is initialized and ready
for use.

CRs-fixed: 2020227
Change-Id: Ibc7a5974adfe803c5b9892986165e2e1e3151fc7
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>