net: phylink: further mac_config documentation improvements

While reviewing the DPAA2 work, it has become apparent that we need
better documentation about which members of the phylink link state
structure are valid in the mac_config call.  Improve this
documentation.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>