usb: gadget: initialize the strings in tcm_usb_gadget properly

I have no idea what I've been thinking while I was doing this in the first
place. Now the strings are initialized properly and reported by lsusb.

Cc: stable@vger.kernel.org # v3.5
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>