net: dsa: microchip: ksz9477: cleanup the ksz9477_switch_detect

The ksz9477_switch_detect performs the detecting the chip id from the
location 0x00 and also check gigabit compatibility check & number of
ports based on the register global_options0. To prepare the common ksz
switch detect function, routine other than chip id read is moved to
ksz9477_switch_init.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>