soundwire: intel: fix intel_suspend/resume defined but not used warning

soundwire: intel: fix intel_suspend/resume defined but not used warning

When CONFIG_PM_SLEEP is not defined, GCC throws compilation warnings:

drivers/soundwire/intel.c:1799:12: warning: ‘intel_resume’ defined but not
used [-Wunused-function]
 static int intel_resume(struct device *dev)
            ^~~~~~~~~~~~
drivers/soundwire/intel.c:1683:12: warning: ‘intel_suspend’ defined but not
used [-Wunused-function]
 static int intel_suspend(struct device *dev)
            ^~~~~~~~~~~~~

Fix by using __maybe_unused macro.

Suggested-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200824133234.28115-1-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>