iwlegacy: check for dma mapping errors

Handle pci_map_page() errors. This fixes "DMA-API: device driver failed
to check map error" warning.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>