usb-storage: fix mistake in Makefile

This patch (as1228) fixes a Makefile error introduced when the
subdrivers in usb-storage were split out into separate modules.  The
intention is that when CONFIG_USB_LIBUSUAL is set, libusual.o and
usual-tables.o should be combined into a single object file (called
usb-libusual).  The current Makefile will instead create two separate
objects, and the result won't load properly.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Alan Jenkins <sourcejedi.lkml@googlemail.com>
Reported-and-tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>