net: dsa: add support for ksz9897 ethernet switch

ksz9477 is superset of ksz9xx series, driver just works
out of the box for ksz9897 chip with this patch.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>