trivial: fix typo "varaible" -> "variable"

This patch was created by

	git grep -l '[vV]araible' | xargs -r -t perl -p -i -e 's/\b([Vv]ar)ai(bles?)\b/$1ia$2/g'

Signed-off-by: Uwe Kleine-Koenig <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>