There is no need to handle optional module request in the driver
when user space tools has that feature for ages.

Replace custom code by MODULE_SOFTDEP() macro to let user space know
that we would like to have the DMA driver loaded first, if any.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>