e1000: ethtool: Add missing dma_mapping_error-call in e1000_setup_desc_rings

After dma_map_single, dma_mapping_error must be called.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>