ath9k_htc: Fix target ready race condition

The ready message from the target could be processed
before the host HW init has completed. In this case,
htc_process_target_rdy() would assume the target has timed
out, when it hasn't. Fix this by checking if the target
has sent the ready message properly.

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