usb:dfu:g_dnl: Refactoring the string definition code for g_dnl driver

The manufacturer and product IDs are dynamically assigned when gadget is
bind.
Now the IDs aren't assigned at struct g_dnl_string_defs definition.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: "Egli, Samuel" <samuel.egli@siemens.com>