infiniband/mlx4: check for mapping error

Since ib_dma_map_single can fail use ib_dma_mapping_error to check
for errors.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Acked-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Doug Ledford <dledford@redhat.com>