ath9k: trivial: reorder rx_tasklet processing

DFS events are reported as PHY errors and need to be processed
with a correct timestamp set before ath9k_skb_preprocess() is
called and the frame is possibly dropped.

This patch puts the rxs->mactime calculation before the skb
is preprocessed to prepare for DFS event reporting.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>