nfp: don't dereference a null nn->eth_port to print a warning

On the case where nn->eth_port is null the warning message
is printing the port by dereferencing this null pointer.
Remove the deference to avoid a crash when printing the
warning message.

Detected by CoverityScan, CID#1426198 ("Dereference after null check")

Fixes: ce22f5a2 ("nfp: separate high level and low level NSP headers")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>