iwlwifi: indicate txpower is off in sysfs

The patch checks if the radio is disabled before displaying the tx power
level. Previously when the txpower was set off show_tx_power still
returned the prior power level. Now it will indicate the power has been
turned off.

Signed-off-by: Jay Sternberg <jay.e.sternberg@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>