net/nfp: make sure MTU is never larger than mbuf size

net/nfp: make sure MTU is never larger than mbuf size

Setting a MTU larger than mbuf size is not supported by the device but
not prohibited by the driver. This change adds a restriction to the
driver to prevent setting an MTU that is too large.

While at it define the minimum MTU in the device information to describe
the complete supported MTU range.

Fixes: d4a27a3 ("nfp: add basic features")
Cc: stable@dpdk.org
Signed-off-by: Peng Zhang <peng.zhang@corigine.com>
Signed-off-by: Chaoyong He <chaoyong.he@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Reviewed-by: Walter Heymans <walter.heymans@corigine.com>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>