greybus: Fix loopback app after rename to gpphy

commit 6d94670 gpbridge: rename 'gpbridge' to 'gbphy' everywhere
missed renaming the loopback test app. So do it too.

Testing done: complie and run loopback test

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>