dmatest: return actual state in 'run' file

The following command should return actual state of the test.
	% cat /sys/kernel/debug/dmatest/run

To wait for test done the user may perform a busy loop that checks the state.
	% while [ $(cat /sys/kernel/debug/dmatest/run) = "Y" ]
	> do
	> 	echo -n "."
	> 	sleep 1
	> done
	> echo

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>