usb: gadget: f_mass_storage: reset endpoint driver data when disabled

Gadgets endpoint driver data is a criteria to judge that
whether the endpoints are in use or not. When gadget gets
assigned an endpoint from endpoint list, they check its
driver data if the driver data is NULL.

If the driver data is not NULL then they regard it as in use.
Therefore all of gadgets should reset their endpoints driver
data to NULL as they are disabled. Otherwise it causes a leak
of endpoint resource.

Signed-off-by: Peter Oh <poh@broadcom.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>