iwlwifi: create device configuration header file

The iwl-shared.h header file will be going away
soon. There isn't much left in it that we keep,
other than the device configuration declarations.
Move those out now to a new iwl-config.h header.
iwl-cfg.h seemed like a possible alternative but
those declarations will later live in the PCIe
transport code.

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>