iwlwifi: clean up declarations

A number of declarations in iwl-core.h should
be in agn specific files, and also rename the
iwl-calib.h file to iwl-agn-calib.h to better
reflect that it belongs to agn.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>