Read max MTU from the ENA device

The device now provides driver with max available MTU value it
can handle.

The function setting MTU for the interface was simplified and reworked
to follow up this changes.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12870