iwlwifi: mvm: ignore LMAC scan notifications when running UMAC scans

If the firmware sends LMAC scan notifications while a UMAC scan is
running, just WARN and ignore it, otherwise the scanning state gets
messed up.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>