OTG Final State Machine calls functions which may sleep.

For example, start_gadget callback implementation can use
usb_gadget_vbus_connect(), whose context: can sleep.

If so, mutex should be used instead of spinlock.
Signed-off-by: NAnton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>