net: dsa: mt7530: fix advertising unsupported 1000baseT_Half

Remove 1000baseT_Half to advertise correct hardware capability in
phylink_validate() callback function.

Fixes: 38f790a8

 ("net: dsa: mt7530: Add support for port 5")
Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>