iwlagn: simplify chain_noise_num_beacons indirection

chain_noise_num_beacons is set and never changes.  Use the #define
rather than 3 levels of indirection.

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>