wl12xx: fix DMA-API-related warnings

On the PandaBoard (omap_hsmmc + wl12xx_sdio) with DMA_API_DEBUG:

 WARNING: at lib/dma-debug.c:930 check_for_stack.part.8+0x7c/0xe0()
 omap_hsmmc omap_hsmmc.4: DMA-API: device driver maps memory fromstack

Signed-off-by: Mircea Gherzan <mgherzan@gmail.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>