altos: Directly compute radio tuning values from frequency

The 8051 compiler doesn't support 64-bit ints, so the old
code used an iterative method in 32-bit values. That could take
a long time when the frequency was mis-entered in Hz or MHz instead of
kHz.

This direct-computation uses 64-bit ints, but takes a fixed amount of
time for any inputs.

Signed-off-by: Keith Packard <keithp@keithp.com>