usb: musb: move function declarations to musb_{host,gadget}.h

Let the function declarations live in the header files they belong to,
which makes it easier to stub them 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>