nfp: check idx is -ENOSPC before using it is an index

idx can be returned as -ENOSPC, so we should check for this first
before using it as an index into nn->vxlan_usecnt[] to avoid an
out of bounds array offset read.

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>