iwlwifi: Fix 52 rate report in rx status

This patch fixes reporting rate in RX packets in 52 band.
The rate was updated from CCK rate index instead of OFDM rate 6M
Most of the patch is collateral clean up

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