sfxge(4): add accessor to whole link status

Add a function which makes an MCDI GET_LINK request and
packages up the results. Currently, the get-link function
is triggered from several entry points which then pass
on or store selected parts of the data. When the driver
needs to obtain the current link state, it is more
efficient to do this in a single call.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18281