iwlwifi: mvm: fix missing cleanup in .start() error path
Cleanup of iwl_mvm_leds was missing in case of error,
resulting in the following warning:
WARNING: at lib/kobject.c:196 kobject_add_internal+0x1f4/0x210()
kobject_add_internal failed for phy0-led with -EEXIST, don't try to register things with the same name in the same directory.