usb: gadget: Fix usb string id allocation

Don't reset next_string_id every time the gadget is enabled, this makes
the next strings allocated overwrite strings allocated at probe time.
Instead, fix rndis not to allocate new string ids on every config bind.

Change-Id: Ied28ee416bb6f00c434c34176fe5b7f0dcb2b2d4
Signed-off-by: Benoit Goby <benoit@android.com>