dmaengine: pxa_dma: remove duplicate const qualifier

commit 4e0def88 upstream.

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Cc: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>