net: phylink: Generate caps and convert to linkmodes separately

If we call phylink_caps_to_linkmodes directly from
phylink_get_linkmodes, it is difficult to re-use this functionality in
MAC drivers. This is because MAC drivers must then work with an ethtool
linkmode bitmap, instead of with mac capabilities. Instead, let the
caller of phylink_get_linkmodes do the conversion. To reflect this
change, rename the function to phylink_get_capabilities.

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