Bluetooth: btusb: mediatek: fix WMT failure during runtime suspend

WMT cmd/event doesn't follow up the generic HCI cmd/event handling, it
needs constantly polling control pipe until the host received the WMT
event, thus, we should require to specifically acquire PM counter on the
USB to prevent the interface from entering auto suspended while WMT
cmd/event in progress.

Fixes: a1c49c43

 ("Bluetooth: btusb: Add protocol support for MediaTek MT7668U USB devices")
Co-developed-by: Jing Cai <jing.cai@mediatek.com>
Signed-off-by: Jing Cai <jing.cai@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>