usb: gadget: legacy: fix error return code in cdc_bind()

If 'usb_otg_descriptor_alloc()' fails, we must return a
negative error code -ENOMEM, not 0.

Fixes: ab6796ae

 ("usb: gadget: cdc2: allocate and init otg descriptor by otg capabilities")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>