dmaengine: usb-dmac: Protect PM-only functions to kill warning

If CONFIG_PM=n:

    drivers/dma/sh/usb-dmac.c:677: warning: ‘usb_dmac_runtime_suspend’ defined but not used
    drivers/dma/sh/usb-dmac.c:688: warning: ‘usb_dmac_runtime_resume’ defined but not used

Protect the unused functions by #ifdef CONFIG_PM to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>