DMA: dmatest: extend the "device" module parameter to 32 characters

With Device Tree a typical DMA controller device name can look like
10000000.dma-controller, which extends the current size of the string,
allocated for this parameter. This patch extends its size from 20 to 32
characters.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>