usb: musb: use musb->port_mode

Initialize the host and gadget subsystems of the musb driver only when
the appropriate mode is selected from platform data, or device-tree
information, respectively.

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