igb/igbvf: cleanup exception handling in tx_map_adv

After removing the skb_dma_map/unmap calls the exception handling in
igb_tx_map_adv is not correct.  The issue is that the count value was not
being correctly handled so as a result we were not rewinding the ring as
back as we should have been.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>