usb: gadget: configfs: add cfg_to_gadget_info() helper

add this helper function can simplify code of
config_usb_cfg_link() and config_usb_cfg_unlink().

Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1634649997-28745-2-git-send-email-quic_linyyuan@quicinc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>