media: cx24120: Add retval check for cx24120_message_send()

If cx24120_message_send() returns error, we should keep local struct
unchanged.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 5afc9a25

 ("[media] Add support for TechniSat Skystar S2")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>