Staging: rts5208: fix check for dma mapping error

use dma_mapping_error() instead of comparing the returned address with
zero after dma_map_single().

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>