usb: musb: fix runtime PM in debugfs

MUSB driver now has runtime PM support, but the debugfs driver misses
the PM _get/_put() calls, which could cause MUSB register access
failure.

Cc: stable@vger.kernel.org # 4.9+
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>