iwlwifi: more RS improvements

Main changes:
1. no need to re-calculate expected_tpt when only toggling antenna
2. changing the code to be more strict on input and status of variables
3. enhanced debug prints
4. move to search table only if improving tpt
       (don't move if SR is improved but tpt is not)

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>