gadget: f_thor: fix hang-up with ctrl-c

After the commit 6aae8476 ("gadget: f_thor: Fix memory leaks of
usb request and its buffer"), there is hang-up with ctrl-c in some
udc. It is because req of out_ep is freed before out_ep is disabled.
Fix hang-up with ctrl-c by disabling ep before free req of the ep.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>