iwlwifi: fix LED stall

This patch fixes LED stall. last_blink_time was updated only if LED command
was sent, causing wrong computation of the througput.
Some code cleanup comes with this patch as well

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>