cxgb3: Check and handle the dma mapping errors

This patch adds checks at approprate places whether *dma_map*() call has
succeeded or not.

Signed-off-by: Santosh Rastapur <santosh@chelsio.com>
Reviewed-by: Jay Fenlason <fenlason@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>