ath9k: fix ready time of the multicast buffer queue

commit 3b3e0efb5c72c4fc940af50b33626b8a78a907dc upstream.

qi->tqi_readyTime is written directly to registers that expect
microseconds as unit instead of TU.
When setting the CABQ ready time, cur_conf->beacon_interval is in TU, so
convert it to microseconds before passing it to ath9k_hw.

This should hopefully fix some Tx DMA issues with buffered multicast
frames in AP mode.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>