MFC r351196, r351324, r351326:

ng_ubt(4): do not attach Intel Wireless 8260/8265 in bootloader mode.

Add helper function for synchronous execution of HCI commands at probe
stage and use this function to check firmware state of Intel Wireless
8260/8265 bluetooth devices found in many post 2016 year laptops.

Attempt to initialize FreeBSD bluetooth stack while such a device is in
bootloader mode locks the adapter hardly so it requires power on/off
cycle to restore.

This change blocks ng_ubt attachment unless operational firmware is
loaded thus preventing the lock up.

PR: 237083
Reviewed by: hps, emax
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21071