usb: musb: fix compilation warning on unused function

The function musb_run_resume_work is called only when CONFIG_PM is
enabled. So this function should not be defined when CONFIG_PM is
disabled. Otherwise the compiler issues a warning:

drivers/usb/musb/musb_core.c:2057:12: error: ‘musb_run_resume_work’ defined but
not used [-Werror=unused-function]
 static int musb_run_resume_work(struct musb *musb)
            ^~~~~~~~~~~~~~~~~~~~

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>