tulip: Properly check dma mapping result

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>