usb: musb: Start using struct usb_otg

Use struct usb_otg members with OTG specific functions instead
of usb_phy members.

[ balbi@ti.com: added a missing change on musb_gadget.c to avoid
	a compile error on a later patch ]

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>