Cleanup dma on stack issues:

- no DMA on stack
- cleanup unclear endianness issue

Corrected version of Oliver Neukum's original patch for at76_usb.
Signed-off-by: NJason Andryuk <jandryuk@gmail.com>
Signed-off-by: NKalle Valo <kalle.valo@iki.fi>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>