ath9k: Fix symbol overlap window for half/quarter channels

Since commit cd6cfd73
"ath9k: do not set half/quarter channel flags in AR_PHY_MODE" the
condition "rfMode & (AR_PHY_MODE_QUARTER | AR_PHY_MODE_HALF)" would
never evaluate to true.

Fix this by using the available IS_CHAN_HALF_RATE and IS_CHAN_QUARTER_RATE
marcros instead.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>