usb: musb: gadget: nuke endpoint before setting its descriptor to NULL

Some functions, such as f_sourcesink, rely on an endpoint's desc
field during their requests' complete() callback, so clear it only
_after_ nuking all requests to avoid NULL pointer dereference.

Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>