greybus: camera: Factorize link power mode configuration code into a function

Avoid duplicating the same code block multiple times.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo.mondi@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>