usb: musb-new: Call musb_platform_exit from musb_stop

musb stop is musb core call during unregister or shutting down
gadget or host musb. For graceful exit add musb_platform_exit
on musb_stop so-that it can exit the musb platform driver as well.

Tested-by: Chen-Yu Tsai <wens@csie.org> # A33-OlinuXino
Tested-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>