w1: ds2482: make module parameter extra_config static

The variable extra_config is local to the source and does not
need to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'extra_config' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>