nfp: flower: fix port metadata conversion bug

Function nfp_flower_repr_get_type_and_port expects an enum nfp_repr_type
return value but, if the repr type is unknown, returns a value of type
enum nfp_flower_cmsg_port_type.  This means that if FW encodes the port
ID in a way the driver does not understand instead of dropping the frame
driver may attribute it to a physical port (uplink) provided the port
number is less than physical port count.

Fix this and ensure a net_device of NULL is returned if the repr can not
be determined.

Fixes: 1025351a ("nfp: add flower app")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>