Bluetooth: btmtk: Fix failed to send func ctrl for MediaTek devices.

Use usb_autopm_get_interface() and usb_autopm_put_interface()
in btmtk_usb_shutdown(), it could send func ctrl after enabling
autosuspend.

Bluetooth: btmtk_usb_hci_wmt_sync() hci0: Execution of wmt command
           timed out
Bluetooth: btmtk_usb_shutdown() hci0: Failed to send wmt func ctrl
           (-110)

Fixes: 5c5e8c52 ("Bluetooth: btmtk: move btusb_mtk_[setup, shutdown] to btmtk.c")
Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>