net: phylink: reimplement population of pl->supported for in-band

phylink_parse_mode() populates all possible supported link modes for a
given phy_interface_t, for the case where a phylib phy may be absent and
we can't retrieve the supported link modes from that.

Russell points out that since the introduction of the generic validation
helpers phylink_get_capabilities() and phylink_caps_to_linkmodes(), we
can rewrite this procedure to populate the pl->supported mask, so that
instead of spelling out the link modes, we derive an intermediary
mac_capabilities bit field, and we convert that to the equivalent link
modes.

Suggested-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>