iwlwifi: set default tx power user limit to minimal

Set the tx_power_user_lmt to the lowest power level
this value will get overwritten by channel's max power avg
from eeprom

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>