greybus: camera: Add missing return value

Add missing return value assignement when changing unipro power mode to
PWM-G1.

Signed-off-by: Jacopo Mondi <jacopo.mondi@linaro.org>
Reviewed-by: Gjorgji Rosikopulos <grosikopulos@mm-sol.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>