usb:g_dnl:dfu: Download gadget and DFU function code clean up

The download gadget code and DFU function lacks of proper declarations
for the case when a target board wants to use only one of available usb
functions.

Moreover the relevant declarations have been moved to consistent
localization (like <dfu.h>).

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>