cxgbev(4): Compare at most 16 bytes of the Ethernet header when trying
to coalesce tx work requests.

Note that Coverity will still treat this as an out-of-bounds access. We
do want to compare 16B starting from ethmacdst but cmp_l2hdr was was
going beyond that by 2B.

cmp_l2hdr was introduced in r362905.

Reported by: Coverity (CID 1430284)
Sponsored by: Chelsio Communications