ath9k_hw: add a missing delay for an analog register

AR_AN_SYNTH9 is in the analog shift register range and thus needs to be
written using the ath9k_hw_analog_shift_rmw function.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>