iwlwifi: parametrize configuration of the PLL for exclusion on 6000

added a config parameter to enable setting PLL_CFG.  older hardware has
this parameter set true.  the 6000 family does not support this setting,
so this parameter set false.

Signed-off-by: Jay Sternberg <jay.e.sternberg@linux.intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>