ath9k: Fix IBSS joiner mode

On joining an existing IBSS network, beaconing has to start
only after a TSF sync has happened by receiving a beacon from
the BSS. In creator mode, beaconing can start immediately after
a HW reset has been done.

Now that mac80211 notifies the driver of the mode type (creator/joiner)
via ieee80211_bss_conf->ibss_creator, make use of it to properly setup
the HW beacon timers.

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