ath9k: Add a simple virtual wiphy scheduler

This is a very simple scheduler that goes through the wiphys and
schedules one at a time every N milliseconds (current default value:
500 ms). This is enough for initial testing, but there are number of
areas where a more complex scheduler can improve operations greatly.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>