V4L/DVB (7924): ivtv/cx18: snprintf fixes

snprinf() takes the trailing \0 into account in its length calculations,
so there is no need to subtract 1 to the buffer size.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>