greybus: loopback_test: Decrease the max number of devices

Its unrealistic to expect 50 loopback devices,
and the mask parameter can hold up to "int" anyways
so decrease to max number of devices to sane value.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>