ASoC: msm8x16-wcd: Avoid null pointer spmi access

SPMI devices for PMIC digital and analog codec
are registered one after other whenever probe happens.
Say first digital slave is registered, it also registers
codec to ALSA core. There is a chance that codec probe
is triggered even before PMIC analog slave device is
registered. In codec probe, when try to access
any SPMI register related to analog slave, it will crash.
Handle this in probe function to avoid the crash.

CRs-Fixed: 969220
Change-Id: If63831774960ccb2f62cc4e9b855ccee0ba6f4ab
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>