iwmbtfw(8): Improve Intel 7260/7265 adaptors handling

• Allow firmware downloading for hw_variant #8;
• Enter manufacturer mode for setting of event mask;
• Handle multi-event response on HCI commands for 7260; This allows to remove kludge with skipping of 0xfc2f opcode.
• Disable patch and exit manufacturer mode on downloading failure;
• Use default firmware if correct firmware file is not found;

Reviewed by: Philippe Michaud-Boudreault <pitwuu_AT_gmail_DOT_com>
MFC after: 1 week
Tested by: arrowd
Differential revision: https://reviews.freebsd.org/D30543