usb: gadget: fix potential double-free in m66592_probe.

m66592_free_request() is called under label "err_add_udc"
and "clean_up", and m66592->ep0_req is not set to NULL after
first free, leading to a double-free. Fix this issue by
setting m66592->ep0_req to NULL after the first free.

Fixes: 0f91349b ("usb: gadget: convert all users to the new udc infrastructure")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Signed-off-by: Felipe Balbi <balbi@kernel.org>