iwlwifi: cleans up scanning code

This patch
1. cleans up scanning code.
2. It adds round robin of TX antannas/chains.

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>