dmaengine/dmatest: terminate transfers only in case of errors

dmatest erroneously terminated transfers in normal cases also leading to
test failures for multiple threads over a channel. Fix this and
terminate transfers only in case of errors.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>