net: mscc: fix the frame extraction into the skb

When extracting frames from the Ocelot switch, the frame check sequence
(FCS) is present at the end of the data extracted. The FCS was put into
the sk buffer which introduced some issues (as length related ones), as
the FCS shouldn't be part of an Rx sk buffer.

This patch fixes the Ocelot switch extraction behaviour by discarding
the FCS.

Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>