[media] cx231xx: Fix memory leak

dma_area needs to be freed when the device is closed.

Based on em23xx-audio.c

Signed-off-by: Jean-Baptiste Theou <jtheou@adeneo-embedded.us>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>