ix(4): Add control of 2.5/5G autonegotiation speeds

This change enables the user to control 2.5G and 5G autonegotiation
speeds via advertise_speed sysctl for X550T devices. Due to reported
interoperability issues with switches, 2.5G and 5G speeds will not be
advertised by default.

Signed-off-by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>
Co-authored-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

Tested by: gowtham.kumar.ks@intel.com
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D26245