[media] bttv: avoid flooding the kernel log when i2c debugging is disabled

When the bttv driver is running without i2c_debug being set, the kernel
log is being flooded with the string ">". This string is really a part of
a debug message that is logged using several substrings protected by a
conditional check.
This patch adds the same conditional check to the leaked substring.

Signed-off-by: John Törnblom <john.tornblom@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>