ath9k: Fix programming SYNTH4 for AR9462

The LONG_SHIFT_SELECT offset is different for AR9462 from
the other chip families. Fix this.

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