wcd934x: Fix NULL pointer dereference for wcd9xxx pointer

During bind/unbind of ngd_msm_ctrl power_gate_digital_core
is called. Simultaneously tavil_remove is also called
making tavil pointer as NULL.  Check for tavil as not NULL
at the start of function to avoid NULL pointer dereference.

CRs-Fixed: 2267796
Change-Id: I6fc476cc8b706b556836f30838983de0f34d4fc1
Signed-off-by: Vatsal Bucha <vbucha@codeaurora.org>