iwlagn: iwl-pci doesn't include iwl-dev any more

Move all the iwlXXX_abgn_cfg forward declaration to a separate file
so that iwl-pci.c doesn't need to include iwl-agn.h that includes
all iwl-dev.h
This allows to provide real encapsulation. Dereferencing iwl_priv
in the bus layer will now lead to a compilation error.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>