usb: gadget: printer: using gadget_is_otg to check otg support at runtime

We need to use gadget_is_otg to check if the gadget is really
otg support at runtime, other composite gadget drivers have already
followed this method.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>