usb: gadget: composite: Fix return value in case of error

In 'composite_os_desc_req_prepare', if one of the memory allocations fail,
0 will be returned, which means success.
We should return -ENOMEM instead.

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