nfp: allow apps to request larger MTU on control vNIC

Some apps may want to have higher MTU on the control vNIC/queue.
Allow them to set the requested MTU at init time.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>