at76c50x-usb: clean up DMA on stack

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: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>