V4L/DVB (6245): GemTek Radio card - frequency calculation

Frequency calculation to use better math.  It's still the same
IF offset and step size (which are not the same as the datasheet says) as
the code was before.  It's just more efficient and accurate.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Reviewed-by: Pekka Seppänen <pexu@kapsi.fi>
Signed-off-by: Douglas Schilling Landgraf <dougsland@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>