nui: Fix dma_mapping_error() check

nui: Fix dma_mapping_error() check

dma_map_XXX() functions return values DMA_MAPPING_ERROR as error values
which is often ~0.  The error value should be tested with
dma_mapping_error().

This patch creates a new function in niu_ops to test if the mapping
failed.  The test is fixed in niu_rbr_add_page(), added in
niu_start_xmit() and the successfully mapped pages are unmaped upon error.

Fixes: ec2deec1f352 ("niu: Fix to check for dma mapping errors.")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>