tsnep: Remove FCS for XDP data path

The RX data buffer includes the FCS. The FCS is already stripped for the
normal data path. But for the XDP data path the FCS is included and
acts like additional/useless data.

Remove the FCS from the RX data buffer also for XDP.

Fixes: 65b28c81 ("tsnep: Add XDP RX support")
Fixes: 3fc23339

 ("tsnep: Add XDP socket zero-copy RX support")
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>