cnss2: add null pointer check

The return value of the function cnss_bus_dev_to_plat_priv() has a
chance to NULL, So we have to check "plat_priv" if it is a null
pointer before it is dereferenced.

Change-Id: I8a5bbe5037d1f15113e58bb16b90f243ea8ef807
Signed-off-by: Ke Huang <keh@codeaurora.org>
Signed-off-by: Kuichu Ni <kuichun@codeaurora.org>