nfp: flower: remove headroom from max MTU calculation

Since commit 29a5dcae ("nfp: flower: offload phys port MTU change") we
take encapsulation headroom into account when calculating the max allowed
MTU.  This is unnecessary as the max MTU advertised by firmware should have
already accounted for encap headroom.

Subtracting headroom twice brings the max MTU below what's necessary for
some deployments.

Fixes: 29a5dcae ("nfp: flower: offload phys port MTU change")
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>