[media] vb2: Fix queue_setup() callback description

Correct meaning of the last sensence by swapping it with previous.
Fix two small typos.

Signed-off-by: Anton Leontiev <scileont@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>