As far as prep_dma() is called with spinlock held,
we have to pass GFP_ATOMIC regardless of gfp argument.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: NFelipe Balbi <balbi@ti.com>