usb: dwc3: ep0: fix debug message

The way it was before was really meaningless.
Now it looks saner.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>