usb: musb: blackfin: fix unused warnings on suspend/resume

When CONFIG_PM_SLEEP is disabled, SIMPLE_DEV_PM_OPS does not use
bfin_resume and bfin_suspend even if CONFIG_PM is enabled:

drivers/usb/musb/blackfin.c:602:12: warning: ‘bfin_resume’ defined but
not used [-Wunused-function]
 static int bfin_resume(struct device *dev)
            ^~~~~~~~~~~
drivers/usb/musb/blackfin.c:585:12: warning: ‘bfin_suspend’ defined but
not used [-Wunused-function]
 static int bfin_suspend(struct device *dev)
            ^~~~~~~~~~~~

The preprocessor condition should be on CONFIG_PM_SLEEP, not on CONFIG_PM.
However it is better to mark these functions as __maybe_unused.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>