usb: musb: factor some host-specific functions

In particular, this introduces musb_host_resume_root_hub()and
musb_host_poke_root_hub() which will be stubbed out later.

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