iwlwifi: fix IBSS beaconing

My previous patch to clean up all RXON handling
inadvertently broke IBSS because it failed to
take into account that unlike in AP mode, IBSS
requires beacons to be sent only after setting
the RXON assoc.

Fix this, clean up the code a bit, improve the
error checking around this, and also react to
beacon changes in IBSS mode from mac80211.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>