ath9k_hw: Fix radio programming for AR9550

For AR9550, program the synth value based on the ref. clock.
The logic for AR9550 is similar to AR9330, but keep the code
separate since changes for AR9330 are required - which would be
done later.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>