ixgbe: Fix ixgbe_tx_map error path

Commit e5a43549

 (ixgbe: remove
skb_dma_map/unmap calls from driver) looks to have introduced a bug in
ixgbe_tx_map. If we get an error from a PCI DMA call, we loop backwards
through count until it becomes -1 and return that.

The caller of ixgbe_tx_map expects 0 on error, so return that instead.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>