8139cp: Add dma_mapping_error checking

Self explanitory dma_mapping_error addition to the 8139 driver, based on this:
https://bugzilla.redhat.com/show_bug.cgi?id=947250



It showed several backtraces arising for dma_map_* usage without checking the
return code on the mapping.  Add the check and abort the rx/tx operation if its
failed.  Untested as I have no hardware and the reporter has wandered off, but
seems pretty straightforward.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>