ath9k: skip ATH9K_INT_TIM_TIMER when we are idle

We should not be idle when we get the ATH9K_INT_TIM_TIMER,
otherwise its a sign of something broken in our design with
our idle state machine and mac80211. Skip these and WARN once
just in case this is triggerable.

Cc: Paul Stewart <pstew@google.com>
Cc: Amod Bodas <amod.bodas@atheros.com>
signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>