drivers/usb/gadget: select objects by obj-$(CONFIG-...)

Before switching to the real Kbuild, drivers/usb/gadget/Makefile
must be fixed.
If none of CONFIG_USB_GADGET, CONFIG_USB_ETHER, CONFIG_USB_DEVICE
is defined, both obj- and obj-y get empty.

We need non-empty obj- or obj-y on each Makefile
to generate built-in.o on the real Kbuild.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>