usb: gadget: composite: Fix function used to free memory

'cdev->os_desc_req' has been allocated with 'usb_ep_alloc_request()' so
'usb_ep_free_request()' should be used to free it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>