iwn: some initialization / RF switch state change fixes.

• Check return code from initialization path; otherwise, vap state

may be wrong after an error.

• Do not try to run iwn_stop() / iwn_init() multiple times.
• Merge iwn_radio_on/off() and move RFKILL bit check into the task.
• Try to handle possible RF switch state change in S3 state (PR 181694).

PR: 181694
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D9797