MFC r339587:
Added support for formula-based arbitrary baud rates, in contrast to
the current fixed values, which enables use of rates above 1 Mbps.
Improved the detection of HXD chips, and the status flag handling as
well.

Submitted by: Gabor Simon <gabor.simon75@gmail.com>
PR: 225932
Approved by: re (kib)
Differential revision: https://reviews.freebsd.org/D16639
Sponsored by: Mellanox Technologies