USB: Correct Makefile to make isp1760 buildable

Signed-off-by: Ivan Kuten <ivan.kuten@promwad.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>