[media] af9033: prevent unintended underflow

As spotted by Dan Carpenter <dan.carpenter@oracle.com> (thanks!), we have
improperly used an unsigned variable in a calculation that may result in a
negative number. This may cause an unintended underflow if the interface
frequency of the tuner is > approx. 40MHz.
This patch should resolve the issue, following an approach similar to what is
used in af9013.c.

[crope@iki.fi: add Reported-by]
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans-Frieder Vogt <hfvogt@gmx.net>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>