lan966x: Fix unloading/loading of the driver

lan966x: Fix unloading/loading of the driver

It was noticing that after a while when unloading/loading the driver and
sending traffic through the switch, it would stop working. It would stop
forwarding any traffic and the only way to get out of this was to do a
power cycle of the board. The root cause seems to be that the switch
core is initialized twice. Apparently initializing twice the switch core
disturbs the pointers in the queue systems in the HW, so after a while
it would stop sending the traffic.
Unfortunetly, it is not possible to use a reset of the switch here,
because the reset line is connected to multiple devices like MDIO,
SGPIO, FAN, etc. So then all the devices will get reseted when the
network driver will be loaded.
So the fix is to check if the core is initialized already and if that is
the case don't initialize it again.

Fixes: db8bcaad5393 ("net: lan966x: add the basic lan966x driver")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230522120038.3749026-1-horatiu.vultur@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>