iwlwifi: move channel switch related functions

With the creation of iwl-agn-devices.c,
iwl-core.c can be cleaned up a bit more by
moving beacon time related functions from
iwl-core.c to iwl-agn-devices.c

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