iwlegacy: more checks for dma mapping errors

This patch check output of pci_map_single() calls. I missed them on
my previous patch "iwlegacy: check for dma mapping errors", which
fixed only pci_map_page() calls.

To handle remaining possible dma mappings errors, we need to rearrange
ilXXXX_tx_skb() and il_enqueue_hcmd() functions.

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