ath9k: call ath9k_hw_set_desc_link for beacon descriptors

This ensures that only ath9k_hw_set_desc_link needs to recalculate the tx
descriptor checksum on AR9380+

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