dmatest: move dmatest_channels and nr_channels to dmatest_info

We don't need to have them global and later we would like to protect access to
them as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>