dmaengine/dmatest: Pass timeout via module params

When we try to test all channels present on our controller together, some
channels of lower priority may be very slow as compared to others. If number of
transfers is unlimited, some channels may timeout and will not finish within 3
seconds. Thus, while doing such regress testing we may need to have higher value
of timeouts. This patch adds support for passing timeout value via module
parameters. Default value is 3 msec, a negative value means max timeout
possible.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>