sfxge: avoid usage of ifm_data

The driver uses ifm_data to save capabilities mask calculated during
initialization when supported phy modes are discovered.
The patch simply calculates it when either media or options are changed.

Reviewed by: glebius
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2540