[media] cx18: fix spelling mistake: "demodualtor" -> "demodulator"

trivial fix to spelling mistake and add in a white space in
a CX18_ERR error message

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>