net: mscc: ocelot: refactor ports parsing code into a dedicated function

mscc_ocelot_probe() is already pretty large and hard to follow. So move
the code for parsing ports in a separate function.

This makes it easier for the next patch to just call
mscc_ocelot_release_ports from the error path of mscc_ocelot_init_ports.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>