wifi: iwlwifi: mvm: check vif for NULL/ERR_PTR before dereference

iwl_mvm_get_bss_vif might return a NULL or ERR_PTR. Some of the callers
check only the NULL case, and some doesn't check at all.

Some of the callers even have a pointer to the mvmvif of the bss vif,
so we don't even need to call this function, and can simply get the vif
from mvmvif. Do it for those cases, and for the others - properly check
if IS_ERR_OR_NULL

Fixes: ec0d43d2

 ("wifi: iwlwifi: mvm: Activate EMLSR based on traffic volume")
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20240703064027.a661f8c65aac.I45cf09b01af8ee3d55828863958ead741ea43b7f@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>