Fix netstat counters mapping in mlx5en(4).

The current mapping of driver counters to netstat counters is wrong.
For example, a single jabber packet, will cause the Ierrs counter to
count three times.

The work for mapping the hardware and software counters to their right
place in netstat counters were already done in Linux, take that as is
to the FreeBSD driver.

Submitted by: slavash@
MFC after: 3 days
Sponsored by: Mellanox Technologies