Implement jumbo frame support in mvneta driver

This patch introduces processing of the frames
up to 9kB by the mvneta driver. Some versions of
this NIC limit TX checksum offloading, depending
on the frame size, so add appropriate handling
of this feature.

Submitted by: Kornel Duleba
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D23225