net: dsa: microchip: move switch chip_id detection to ksz_common

KSZ87xx and KSZ88xx have chip_id representation at reg location 0. And
KSZ9477 compatible switch and LAN937x switch have same chip_id detection
at location 0x01 and 0x02. To have the common switch detect
functionality for ksz switches, ksz_switch_detect function is
introduced.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>