usb: musb: Rework USB and USB_GADGET dependency

This USB controller can work in as host-only, gadget-only or dual-role
modes. Rework the dependency on the USB and USB_GADGET configs in order
to allow building the driver when !USB or !USG_GADGET.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>