Fix "received NULL mbuf" bug in VNIC

Do not modify NIC_QSET_CQ_0_7_HEAD manually, especially
in non-atomic context.
It doesn't seem to be necessary to recreate CQ head after
interrupt clearing too.

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5533