fix ht/40 configuration for ar9331 (hornet).

The synth programming here requires the real centre frequency,
which for HT20 channels is the normal channel, but HT40 is
/not/ the primary channel. Everything else was using 'freq',
which is the correct centre frequency, but the hornet config
was using 'ichan' to do the lookup which was also the primary
channel.

So, modify the HAL call that does the mapping to take a frequency
in MHz and return the channel number.

Tested:

• Carambola 2, AR9331, tested both HT/20 and HT/40 operation.