iwlwifi: add REPLY_TX_POWER_DBM_CMD to get_cmd_string

This patch adds REPLY_TX_POWER_DBM_CMD to get_cmd_string.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
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>