iwlwifi: 3954 renames iwl3945_rate_scale_priv to iwl3945_rs_sta

This patch renames iwl3945_rate_scale_priv to iwl3945_rs_sta as it
better represents the purpose of this variable.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>