[media] mceusb: remove pointless mce_flush_rx_buffer function

This function just submits the urb much like mceusb_dev_resume; removing
it simplifies mce_request_packet.

Also add missing usb_kill_urb to mce_dev_probe.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>