Selecting 'Custom' for cache level in simple preferences should leave current
settings alone.  Custom levels are set in advanced preferences.  Fixes #5868.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit fa4b920d664728768c3d15fe7ffc0f0a348cce9e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>