libertas: Add missing sentinel at end of if_usb.c fw_table

This sentinel tells the firmware loading process when to stop.

Reported-and-tested-by:  <syzbot+98156c174c5a2cad9f8f@syzkaller.appspotmail.com>
Signed-off-by: Kevin Easton <kevin@guarana.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>