V4L/DVB (7804): tea5767: Fix error logic

As pointed by Andrew Morton, the error testing were wrong. After reviewing
tea5767, it were returning a positive value for errors.

So, the double errors were cancelling each other.

This patch fix it properly. It also considers any positive value as ok, on
tuner-core.


Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>