linux: fix 3.8.x stable version

Commit c461e642 bumped the linux 3.8.x
stable version from 3.8.5 to 3.8.6, but did so in only one of the two places
in which the version number is hard-coded in linux/Config.in

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>