ipw2x00: add checks for dma mapping errors

ipw2100_alloc_skb() and ipw2100_tx_send_data() do not check if mapping
dma memory succeed. The patch adds the checks and failure handling.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>