iwlagn: rename iwlagn module iwlwifi and alias to iwlagn.

Rename the iwlagn module as iwlwifi in preparation for future
changes.  Add an alias to iwlagn for backward compatibility.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>