vmxnet3: fix avail count on txq_gc

txq_encap decrements txq.avail for each mbuf, so we need to increment same number here.

Reviewed-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>