dmatest: make module parameters writable

The debugfs interface brought a copy of the test case parameters. This makes
different set of values under /sys/module/dmatest/parameters/ and
/sys/kernel/debug/dmatest/. The user might be confused by the divergence of
values.

The proposed solution in this patch is to make module parameters writable and
remove them from the debugfs. Though we're still using debugfs to control test
runner and getting results.

Documentation part is updated accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Suggested-by: Dan Williams <djbw@fb.com>
Signed-off-by: Dan Williams <djbw@fb.com>