Tulip throws an error when dma debugging is enabled, as it doesn't properly
check dma mapping results with dma_mapping_error() durring tx ring refills.
Easy fix, just add it in, and drop the frame if the mapping is bad
Signed-off-by: Neil Horman <nhorman@tuxdriver.com> CC: Grant Grundler <grundler@parisc-linux.org> CC: "David S. Miller" <davem@davemloft.net> Reviewed-by: Grant Grundler <grundler@parisc-linux.org> Signed-off-by: David S. Miller <davem@davemloft.net>