nfp: prevent theoretical buffer overrun in nfp_eth_read_ports

Prevent theoretical buffer overrun by returning an error if
the number of entries returned by the firmware does not match those
present.

Also use a common handling error path.

Found by inspection.

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