usb: convert to partial linking

Looks like this was missed during the conversion to partial linking.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/Makefile b/Makefile
index 6bb5494..e765453 100644
--- a/Makefile
+++ b/Makefile
@@ -266,7 +266,7 @@
 LIBS += drivers/rtc/librtc.o
 LIBS += drivers/serial/libserial.o
 LIBS += drivers/twserial/libtws.o
-LIBS += drivers/usb/eth/libusb_eth.a
+LIBS += drivers/usb/eth/libusb_eth.o
 LIBS += drivers/usb/gadget/libusb_gadget.o
 LIBS += drivers/usb/host/libusb_host.o
 LIBS += drivers/usb/musb/libusb_musb.o