dmaengine: shdma: fix a regression: initialise DMA channels for memcpy

A recent patch has introduced a regression, where repeating a memcpy
DMA test with shdma module unloading between them skips the DMA channel
configuration. Fix this regression by always configuring the channel
during its allocation.

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