dmaengine: qcom-bam-dma: add __maybe_unused annotations for PM

The bam_dma driver gained runtime PM support, but that causes build
warnings whenever CONFIG_PM is disabled:

drivers/dma/qcom/bam_dma.c:1324:12: error: 'bam_dma_runtime_resume' defined but not used [-Werror=unused-function]
 static int bam_dma_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~
drivers/dma/qcom/bam_dma.c:1315:12: error: 'bam_dma_runtime_suspend' defined but not used [-Werror=unused-function]
 static int bam_dma_runtime_suspend(struct device *dev)

This removes the incomplete #ifdef guard and instead marks all
four PM functions as __maybe_unused, which avoids this kind of
warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 7d254559 ("dmaengine: qcom-bam-dma: Add pm_runtime support")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>