net: mscc: allow extracting the FCS into the skb

This patch adds support for the NETIF_F_RXFCS feature in the Mscc
Ethernet driver. This feature is disabled by default and allow a user
to request the driver not to drop the FCS and to extract it into the skb
for debugging purposes.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>