usb: gadget: rndis: don't duplicate the "i" variable

If CONFIG_USB_GADGET_DEBUG_FILES is set then a block is opened and inside
it there is a local variable "i" which hides the "i" local to the
rndis_deregister(). Consequently, a random value is formatted
into the "name" buffer.

This patch removes the block-local i.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>