staging: vt6656: main_usb.c Replace DEVICE_PARAM RxDescriptors0

Replace with vnt_rx_buffers using module_param_named and
MODULE_PARM_DESC

Check user doe not set rx_buffer beyond the define limits of driver

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>