net: dsa: mt7530: Refine message in Kconfig

Refine message in Kconfig with fixing typo and an explicit MT7621 support.

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>