iwmbtfw(8): Ignore unexpected HCI events

If Intel firmware is already in operational mode at boot that takes
place at warm boot, BT adaptor can generate extra HCI events which
interferes with firmware mode detection logic. Ignore them.

Sponsored by: Future Crew LLC
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D46737

(cherry picked from commit aa0b938434a8af8eebf8f2634914f2d9fe8a5dc4)