tg3: Cleanup transmit error path

This patch consolidates the skb cleanup code into a function named
tg3_skb_error_unmap().  The modification addresses a long-standing bug
where pci_unmap_single() was incorrectly being called instead of
pci_unmap_page() in tigon3_dma_hwbug_workaround().

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>