iwlwifi: move utility functions out of iwl-core.h

Move these functions to iwl-agn.h as part of
iwl-core.h cleanup.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>