usb: gadget: tcm_usb_gadget: NULL terminate the FS descriptor list

The descriptor list for FS speed was not NULL terminated. This patch
fixes this.

While here one of the twe two bAlternateSetting assignments for the BOT
interface. Both assign 0, one is enough.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>