ASoC: wcd9320: Fix null pointer access for event callback

The ASoC machine driver provides callback to codec driver to setup
certain settings that are specific to the machine. It is possible that
machine driver does not provide this callback when there are no machine
specific settings. In such cases the callback will be NULL. Make sure to
not access the callback if it is NULL to avoid null pointer dereference.

Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>