iwlwifi: send TXPOWER command after a new RXON command

The patch fixes the problem that TXPOWER command is not sent
after we issue a new RXON command which requires a tune. Otherwise
we won't be able to Tx any frames.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>