net: phylink: Export phylink_caps_to_linkmodes

This function is convenient for MAC drivers. They can use it to add or
remove particular link modes based on capabilities (such as if half
duplex is not supported for a particular interface mode).

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