dpaa2-eth: send a scatter-gather FD instead of realloc-ing

Instead of realloc-ing the skb on the Tx path when the provided headroom
is smaller than the HW requirements, create a Scatter/Gather frame
descriptor with only one entry.

Remove the '[drv] tx realloc frames' counter exposed previously through
ethtool since it is no longer used.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>