iwlwifi: load firmware asynchronously before mac80211 registration

At the wireless summit in Portland we discussed a way of
loading firmware asynchronously from ->probe() before
registration to mac80211, in order to register with the
wireless subsystems with complete information in cases
where firmware is required to know parameters.

This is not yet the case in iwlwifi, but for some new
features we're working on it will be the case since
those will only be supported by new firmware images.

Hence, to start with, convert iwlwifi to load firmware
asynchronously from probe, unbinding the device when
firmware loading fails, and only registering with the
wireless subsystems after firmware has been loaded
successfully.

Future patches will hook into this to register the
new firmware capabilities, depending on the firmware
API version.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>