axgbe: fix some link related issues

By default, axgbe driver does a receiver reset after predefined number
of retries for the link to come up. However, this receiver reset
doesn't always suffice, due to an hardware issue.
In that case, as a workaround, a complete phy reset is necessary.
This patch introduces a sysctl that can be set to 1 to let the driver
reset the phy completely, rather than just doing receiver reset.
The workaround will be removed once the issue is fixed by means
of firmware update.

This patch also fixes the handling of the direct attach cables
properly.

Submitted by: rajesh1.kumar_amd.com
Differential Revision: https://reviews.freebsd.org/D28266