usb: gadget: Use dbg_ep0() macro instead of serial_printf()

All debug messages from ep0.c except a few are printed by dbg_ep0() macro.
So for remaining few exceptions use also dbg_ep0() instead of serial_printf().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Pavel Machek <pavel@ucw.cz>