staging: most: v4l2-aim: remove unnecessary retval

The function aim_register_videodev() uses the variables 'ret' and
'retval' to represent the same value.

This patch removes 'retval' and replaces it with 'ret'. Further, it
replaces the constant return value '-ENODEV' with the result returned by
function video_register_device() in the event something went wrong.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>