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

[ Upstream commit 764f3f1ecffc434096e0a2b02f1a6cc964a89df6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>