mac80211: call ieee80211_recalc_idle() after sending packets

Some drivers (e.g. ath9k) assume that it's safe to go into low-power mode
immediately after the idle state changes. To support that, mac80211 even
calls drv_flush() before that happens.
In some instances, mac80211 sent a packet right after recalculating the
idle state, this patch fixes that.

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