iwlwifi: refactor setting tx power

This patch
1. Refactors settings of tx power
2. enables iwconfig txpower <value>
3. adds 5000 HW tx power

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>