ixl: verify flowid is valid before setting rsstype & flowid

According to section 8.3.2.2.1 of the XL710 datasheet,
FLTSTAT indicates if RSS hashing was used (or flow director
or nothing) to arrive at the flowid it is reporting.
We need to check this before reporting the flowid and setting the
rss type. Not checking this allows invalid flowids (0s) to be
reported for some TCP traffic (it seems to mostly impact incoming
connections). This leads to the inp flowid being 0, and egress
traffic being unbalanced (going to tx queue 0, and NIC 0 of an
lacp bundle).

Differential Revision: https://reviews.freebsd.org/D54442
Reviewed by: adrian, kbowling
Sponsored by: Netflix
MFC after: 2 weeks