via-velocity: fix memory corruption when changing the mtu

velocity_rx_refill() only replenishes the descriptor entries which
belong to the CPU. It works great in the Rx path but the driver must
ensure that all the descriptors are freed before velocity_rx_refill()
is used in velocity_change_mtu(). The patch resets the Rx descriptors
in velocity_free_rd_ring().

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>