usb: gadget: fix error return code in configfs_composite_bind()

Fix to return a negative error code in the go through all configs error
handling case instead of 0(usb_add_function() will overwrite ret to 0).
Also use error code from usb_gstrings_attach() in all strings init error
case instead of -EINVAL.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>