netxen: fix rx dropped stats

Don't count rx dropped packets based on return value of netif_receive_skb(),
which is misleading.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Tested-by: Vernon Mauery <mauery@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>