iwlagn: move context init after firmware loading

The availability of contexts depends on the
firmware capabilities. Currently only the
presence of the second context depends on it,
but soon P2P support will also be different.
Move the context initialisation code to the
firmware-dependent setup before registering
with mac80211 to make it easier to handle.

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>