iwlwifi: mvm: check ARRAY_SIZE(mvm->fw_id_to_mac_id) = IWL_MVM_STATION_COUNT

Since we use IWL_MVM_STATION_COUNT all over the driver, we
need to make sure that it is the right constant to look at.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>