net: phy: mediatek-ge-soc: follow netdev LED trigger semantics

Only blink if the link is up on a LED which is programmed to also
indicate link-status.

Otherwise, if both LEDs are in use to indicate different speeds, the
resulting blinking being inverted on LEDs which aren't switched on at
a specific speed is quite counter-intuitive.

Also make sure that state left behind by reset or the bootloader is
recognized correctly including the half-duplex and full-duplex bits as
well as the (unsupported by Linux netdev trigger semantics) link-down
bit.

Fixes: c66937b0 ("net: phy: mediatek-ge-soc: support PHY LEDs")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>