dmatest: add a 'wait' parameter

Allows for scripting test runs by module load / unload.  Prevent module
load from returning until 'iterations' (finite) tests have completed, or
cause reads of the 'wait' parameter in sysfs to pause until the tests
are done.

Also killed the local waitqueue since we can just let the thread exit
naturally as long as we hold a reference.

Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>