dmaengine: dmatest: Add transfer_size parameter

Existing transfer size "len" is either generated randomly or set to the
size of test_buf_size. In some cases we need to explicitly specify a
transfer size that is different from the buffer size and non aligned to
test the target device's ability to handle unaligned transfers.

This patch adds optional parameter "transfer_size" to allow setting
explicit transfer size for dma transfers.

Signed-off-by: Seraj Alijan <seraj.alijan@sondrel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>