usb: musb: move musb_start to musb_virthub.c

This function has its only user in musb_virthub.c, so let's move it
there and make it static.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>