usb: gadget: Clean up Makefile ifdeffery

Take the USB_ETHER ifdef block apart and make use of obj-$(VAR) instead
to include the source files in build. The duplicate CI_UDC entry is now
removed, the USB_DEVICE ifdef is now reduced to core.o ep.o addition,
the ether.o can be conditionally compiled in using USB_ETHER.

No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>