mfd: wcd9xxx: Prevent null pointer dereference

In cases where some of the required device tree properties are not
available or invalid, it is possible there could be null pointer
dereference to access the data. Prevent null pointer dereference and
handle the error gracefully.

Change-Id: If752ec5faf3216dc12028daa6b927afd5e00d8ab
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>