iwlwifi: beacon format related helper function

Move the ucode beacon formation related helper function from 3945 to
iwlcore, so both _3945 and _agn devices can utilize those functions.

When driver pass the beacon related timing information to uCode in both
spectrum measurement and channel switch commands, the beacon timing
parameter require in uCode beacon format; those helper functions will do
the conversation from uSec to the correct uCode format

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>