usb: gadget: use usb_string_ids_tab instead multiple usb_string_id()

Using usb_string_ids_tab() instead multiple calls of usb_string_id()
seems to be handy. It also allows to add string without many checks.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>