nfp: flower: fix size_t compile warning

A recent addition to NFP introduced a function that formats a string with
a size_t variable. This is formatted with %ld which is fine on 64-bit
architectures but produces a compile warning on 32-bit architectures.

Fix this by using the z length modifier.

Fixes: a6156a6ab0f9 ("nfp: flower: handle merge hint messages")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>