Fix bug in VNIC causing phony number of available TX descriptors

[zbb]

Summary

TSO packets will signal segments TX completion in the separate CQ
descriptors. Each CQ descriptor for HW TSO will point to the same
SQ entry.
Do not invoke nicvf_put_sq_desc() for secondary segments to avoid
free_cnt corruption and eventually integer overflow that will result
in the negative free_cnt value and hence impossibility of further
transmission.

[wma]

Good catch!