USB: f81232: implement RX bulk-in EP

The F81232 bulk-in is RX data + LSR channel, data format is
[LSR+Data][LSR+Data]..... , We had implemented in f81232_process_read_urb().

Signed-off-by: Peter Hung <hpeter+linux_kernel@gmail.com>
[johan: reword comment in process_read_urb ]
Signed-off-by: Johan Hovold <johan@kernel.org>