nfp: cast sizeof() to int when comparing with error code

sizeof() will return unsigned value so in the error check
negative error code will be always larger than sizeof().

Fixes: a0d8e02c ("nfp: add support for reading nffw info")

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>