Rework the input frequency calculation so that it produces the right values
when the ADC oversamples the IF input.

This means MT352 devices can now process a near-zero IF (according to the,
specs 4.57MHz is supported with the default crystal).
Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>