V4L/DVB (13107): tda18271: fix overflow in FM radio frequency calculation

Multiplication by 62500 causes an overflow in the 32 bit freq variable,
which is later divided by 1000 when using FM radio.

This patch prevents the overflow by scaling the frequency value correctly
upfront.  Thanks to Henk Vergonet for spotting the problem and providing
a preliminary patch, which this changeset was based upon.

Cc: Henk Vergonet <Henk.Vergonet@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
CC: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>