usb: dwc3: drop CONFIG_USB_DWC3_DEBUG

now that we have no users of dev_dbg() in dwc3,
we can safely remove CONFIG_USB_DWC3_DEBUG.

If dev_dbg() is ever strictly necessary - and I
don't see why it would, considering we want to
rely on tracepoints for debug - we will depend
on DYNAMIC_PRINTK to enable such messages.

Signed-off-by: Felipe Balbi <balbi@ti.com>