USB: gadget: f_fs: Release endpoint upon disable

Endpoints are claimed using usb_ep_autoconfig function,
It will choose an unclaimed usb_ep and prevent the endpoint
from being returned by a later autoconfig calls. We can mark
the driver_data pointer once ep_enable is done in bind.

If we cannot mark to null upon function disable the corresponding
endpoint is not allocated by a later autoconfig call. The current
code does not make the ep->driver_data to null upon function disable.
This is leading to unclaimed endpoints for later autoconfig calls.
Claim the endpoints by assigning ep->driver_data to NULL.

CRs-Fixed: 633673
Change-Id: I221b98ef36cc2a60d27507a2442061a30ed410f4
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>