iwlagn: introduce iwl-shared.h

It will hold declaration of functions and forward declaration of struct that
are used by several layers.
This will allow modules not to include iwl_priv. iwl_bus and iwl_trans are
still visible to all.

All the layers share the module parameters, move the struct to iwl-shared.h.
Also add all module parameters to iwl_mod_params instead of having them as
global static. This includes

* debug_level
* ant_coupling
* bt_ch_announce
* wanted_ucode_alternative

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>