ath9k: fix duration calculation for non-aggregated packets

When not aggregating packets, fi->framelen should be passed in as length
to calculate the duration. Before the tx path rework, ath_tx_fill_desc
was called for either one aggregate, or one single frame, with the
length of the packet or the aggregate as a parameter.
After the rework, ath_tx_sched_aggr can pass a burst of single frames to
ath_tx_fill_desc and sets len=0.
Fix broken duration calculation by overriding the length in ath_tx_fill_desc
before passing it to ath_buf_set_rate.

Cc: stable@vger.kernel.org
Reported-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>