Bluetooth: btrtl: rename USB fw for RTL8761

According Realteks own BT drivers firmware RTL8761B is for UART
and RTL8761BU is for USB.

Change existing 8761B to UART and add an 8761BU entry for USB

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>