iwl3945: do not use agn specific IWL_RATE_COUNT

Only use IWL_RATE_COUNT_3945 in 3945 code.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>