wifi: iwlwifi: mvm: fix check in iwl_mvm_sta_fw_id_mask

In the previous commit, I renamed the variable to differentiate
mac80211/mvm link STA, but forgot to adjust the check. The one
from mac80211 is already non-NULL anyway, but the mvm one can
be NULL when the mac80211 isn't during link switch conditions.
Fix the check.

Fixes: 2783ab50 ("wifi: iwlwifi: mvm: select STA mask only for active links")
Reviewed-by: Daniel Gabay <daniel.gabay@intel.com>
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240325180850.e95b442bafe9.I8c0119fce7b00cb4f65782930d2c167ed5dd0a6e@changeid


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