iwlwifi: add value and range define for link quality command

Instead of hardcoding the link quality parameters inside the functions,
adding #define in iwl-commands.h to shared by different functions.
Also include the valid range for number of link quality parameters.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>