net: mv643xx_eth: Count dropped packets properly

This commit fixes the current dropped packet count by doing it properly,
increasing the count when a packet is discarded; i.e. the packet is not
processed and the driver returns NETDEV_TX_OK.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>