usb: gadget: gadgetfs: fix an oops in ep_write()

We try to free an ERR_PTR on this error path.

Fixes: b44be246 ('usb: gadget: gadgetfs: Free memory allocated by memdup_user()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>