net: dsa: microchip: common menuconfig for ksz series switch

This patch replaces the two different menuconfig for ksz9477 and ksz8795
to single ksz_common. so that it can be extended for the other switch
like lan937x. And removes the export_symbols for the extern functions in
the ksz_common.h.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>