dmaengine: idma64: Switch to use __maybe_unused instead of ifdeffery

ifdeffery is prone to errors and makes code harder to read.
Switch to use __maybe_unused instead of ifdeffery.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20201104103131.89907-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>