net: phy: Update documentation for get_rate_matching

Now that phylink no longer calls phy_get_rate_matching with
PHY_INTERFACE_MODE_NA, phys no longer need to support it. Remove the
documentation mandating support.

Fixes: 7642cc28

 ("net: phylink: fix PHY validation with rate adaption")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>