nfp: add mutex protection for the port list

We will want to unregister netdevs after their port got reconfigured.
For that we need to make sure manipulations of port list from the
port reconfiguration flow will not race with driver's .remove()
callback.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>