iwlagn: push knowledge of ucode image lower down

Move the knowledge of the ucode image to lower level routines.  Also do
not pass the iwl_priv structure lower than it needs to be.

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>