net: macb: add support for padding and fcs computation

For packets with computed IP/TCP/UDP checksum there is no need to tell
hardware to recompute it. For such kind of packets hardware expects the
packet to be at least 64 bytes and FCS to be computed.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>