sh: prevent the DMA driver from unloading, while in use

This prevents the driver from unloading, while it is in use. Unloading of the
driver, while its DMA channels are held, leads to a kernel Oops.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>