usb: musb: call musb_port_suspend from musb_bus_suspend

Make musb_port_suspend() externally available, and call it when to host
goes into suspend. This allows the core to go into suspend while a
device is connected.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>