dmaengine: shdma: Common SH_DMA_SLAVE_NUMBER

Move SHDMA_SLAVE_NUMBER from asm/dmaengine.h to
shdma.h. Set it to 256 to support a wide range
of processors. The amount of memory consumed by
this change is limited to 256 bits.

While at it, rename to SH_DMA_SLAVE_NUMBER to
match with the rest of the file.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>