In original code the old RX DMA buffer is unmapped and processed and at the end
of the isr a new buffer is mapped with pci_map_single and attached to the RX
descriptor.

If pci_map_single fails then the RX descriptor remains with no valid DMA buffer
attached.
In this condition the DMA will target where it shouldn't with obvious evil
consequences.

Simply avoiding re-arming the descriptor will prevent buggy DMA but it will
result soon in RX stuck.

This patch move the DMA mapping of the new buffer at the beginning of the ISR
(and it adds error check for pci_map_single success/fail).

If the DMA mapping fails then we do not unmap the old buffer and we re-arm the
descriptor without processing it, with the old DMA buffer still attached.

In this way we lose the currently RX-ed packet, but whenever next calls to
pci_map_single will succeed again,then the RX process will go on without stuck.
Signed-off-by: andrea merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>