Replace hard coded constants with self-explanatory names, i.e.
use NSEC_PER_USEC for debounce calculus.

While here, add a unit suffix to debounce period constant.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>