usb: dwc3: dwc3-omap: return -EPROBE_DEFER if probe has not yet executed

return -EPROBE_DEFER from dwc3_omap_mailbox in dwc3-omap.c, if the probe of
dwc3-omap has not yet been executed or failed.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>