iwlwifi: mvm: convert ucode_loaded to a status bit

Convert ucode_loaded to a status bit called FIRMWARE_RUNNING.
This will make it easier to clear this earlier, to avoid any
spurious accesses while shutting down, for example through
debugfs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>