iwlwifi: make index unsigned int for iwl_send_led_cmd

This is a small fix to change the idx type from int to unsigned.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>