iwlwifi: replace magic constants with define

use IWL_CCK_RATES_MASK and IWL_OFDM_RATES_MASK instead of
their values directly.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
cc: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>