[iwm] Handle AUTH->SCAN/INIT and ASSOC->SCAN/INIT better

• Tear down the relevant firmware state (i.e. the station, the vif binding) in these transition cases.

• Before this case would leave the firmware state lying around, resulting in errors and firmware panics in the subsequent association attempts.

Obtained from: dragonflybsd.git 94b501399fde6368ae388a669c95b099a6e66e93