ARM: dmabounce: simplify dma_set_mask()

Simplify the dmabounce specific code in dma_set_mask().  We can just
omit setting the dma mask if dmabounce is enabled (we will have already
set dma mask via callbacks when the device is created in that case.)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>