iwlwifi: make mac80211 ops a device config

In the future, 4965 and modern AGN devices will
need to have different mac80211 callbacks since
they have different capabilities. Prepare for
that by making the mac80211 operations a device
config.

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>