Cleanups to IF-finetuning code.

Make the tuner interface return the raw LO frequency so we can infer
the necessary IF value to program the R2832 with without having to know
ahead of time what IF offset the tuner uses. Remove R82XX_IF_FREQ since
it's no longer needed.

Round the IF divisor in rtlsdr_set_if_freq.

When calculating the actual tuner frequency, round it correctly.

Compute the effective tuned frequency and feed that back to the application
via rtlsdr_get_center_freq on request.

Try to handle various corner cases (direct sampling, offset mode) correctly.
I don't have devices to test this so YMMV.