staging: octeon: disable rx interrupts in oct_rx_shutdown

Disable RX interrupts in oct_rx_shutdown(). This way we don't need to
expose the RX IRQ numbers outside the RX module.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>