iwlagn: replace beacon_time_fsf_bits variable with #define

All devices use the same value for beacon_time_tsf_bits.  Use the #define

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>