dmaengine: dmatest: add short-hand `buf_size` var in dmatest_func()

This is just a cosmetic change, since this variable gets used quite a bit
inside the dmatest_func() routine.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>