sfxge: implement SIOCGI2C to read information from phy modules

The IOCTL is used by 'ifconfig -v' to show SFP+/QSFP+ information
including inventory information and dianostics (temperature, light
levels, voltage etc).

Reviewed by: gnn,melifaro
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D5240