be2net: add dma_mapping_error() check for dma_map_page()

The driver does not check value returned by dma_map_page. The patch
fixes this.

v2: Removed the bugfix for non-bug ;-) (thanks Sathya)

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Sathya Perla <Sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>