net: phylink: Adjust advertisement based on rate matching

This adds support for adjusting the advertisement for pause-based rate
matching. This may result in a lossy link, since the final link settings
are not adjusted. Asymmetric pause support is necessary. It would be
possible for a MAC supporting only symmetric pause to use pause-based rate
adaptation, but only if pause reception was enabled as well.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>