net: dsa: microchip: remove NET_DSA_TAG_KSZ_COMMON

Remove the superfluous NET_DSA_TAG_KSZ_COMMON and just use the existing
NET_DSA_TAG_KSZ. Update the description to mention the three switch
families it supports. No functional change.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>