Fix deadlock in VNIC when using single CPU only

Number of free Tx descriptors does not need to be locked since
it can be modified atomically between SND and CQ tasks.
It will also block Tx routine from sending packets while CQ will not
be able to free descriptors.

Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D6266