tg3: Add more PCI DMA map error checking

This patch adds code to check the status of pci_map_single() before
allowing rx buffers to be used.  It also converts the pci_map_single()
call in tg3_run_loopback() to use skb_dma_map() instead.

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