opencores_eth: fix RX path: FCS, padding and TL

opencores_eth: fix RX path: FCS, padding and TL

OpenCores 10/100 Mbps Ethernet MAC specification doesn't clearly state
whether FCS is counted in the RX frame length or not. Looks like it is.
Append zero FCS to the received frames.

Get rid of big static buffer for RX frame padding, optimize it for the
most common MINFL value range.

Set RXD_TL for the long frames only when HUGEN bit is set.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>