sfxge(4): query and use current MTU if setting the MTU fails

This allows the driver to fall back to the largest usable MTU if a
user attempts to configure an unprivileged function with an MTU higher
than that of the attached port.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6387