wifi: iwlwifi: mvm: fix access to fw_id_to_mac_id

RCU protected fw_id_to_mac_id can be initialized with either
an error code or NULL. Thus, after dereferencing need to check
the value with IS_ERR_OR_NULL() and not only that it is not NULL.
Fix it.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230514120631.ec5f2880e81c.Ifa8c0f451df2835bde800f5c3670cc46238a3bd8@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>