nfp: inherit the max_mtu from the PF netdev

The PF netdev is used for data transfer for reprs, so reprs inherit the
maximum MTU settings of the PF netdev.

Fixes: 5de73ee4 ("nfp: general representor implementation")
Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>