[media] tw686x: silent -Wformat-security warning

Using sprintf() with a non-literal string makes some compiler complain
when building with -Wformat-security (eg. clang reports "format string
is not a string literal (potentially insecure)").

Here sprintf() format parameter is indirectly a literal string so there
is no security issue.  Nevertheless adding a "%s" format string to
silent the warning helps to detect real bugs in the kernel.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>