nfp: add port from netdev validation for EEPROM access

Setting of the port flag `NFP_PORT_CHANGED`, introduced
to ensure the correct reading of EEPROM data, causes a
fatal kernel NULL pointer dereference in cases where
the target netdev type cannot be determined.

Add validation of port struct pointer before attempting
to set the `NFP_PORT_CHANGED` flag. Return that operation
is not supported if the netdev type cannot be determined.

Fixes: 4ae97cae ("nfp: ethtool: fix the display error of `ethtool -m DEVNAME`")
Signed-off-by: Jaco Coetzee <jaco.coetzee@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>