iwlwifi: rate scale module cleanups

This patch does following changes
1. rs_get_expected_tpt_table: changed get to set
2. changed IWL_DEBUG_HT to IWL_DEBUG_RATE as appropriate
3. changed rs_get_supported_rates to be returning value function

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