[PATCH] iwlwifi: rs_rate_scale_perform clean up

This patch cleans up rs_rate_scale_perform function. It removes dead
code, shortens variable names and removes useless return i.e. function
now returns void.

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>