iwm - Track firmware state better, and improve handling in iwm_newstate().

iwm - Track firmware state better, and improve handling in iwm_newstate().

* This avoids firmware resets in all the cases in iwm_newstate(). Instead
  iwm_bring_down_firmware() is called, which tears down all the STA
  connection state, according to the sc->sc_firmware_state value.

* Improve the behaviour of the LED blinking a bit, so it only blinks when
  there really is a wireless scan going on.

* Print the newstate arg in debug output of iwm_newstate(), to help in
  debugging.

This is inspired by the firmware state maintaining change in OpenBSD's iwm, by
stsp@openbsd.org (OpenBSD Git 0ddb056fb7370664b1d4b84392697cb17d1a414a).