net: phy: leds: Don't make our own link speed names

The phy core provides a handy phy_speed_to_str() helper, so use that
instead of doing our own formatting of the different known link speeds.
To do this, increase PHY_LED_TRIGGER_SPEED_SUFFIX_SIZE to 11 so we can fit
'Unsupported' if necessary.

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>