Documentation/DMA-mapping.txt: update for pci_dma_mapping_error() changes

Make the example code consistent with changed API.

Signed-off-by: Marin Mitov <mitov@ispp.bas.bg>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>