iwlagn: move iwl_beacon_time_mask_XXX near to usage

Since iwl_beacon_time_mask_[high,low] are used in iwl-core.c only,
move them to there.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>