iwlwifi: add possibility to disable tx_power calibration

This patch adds the possibility to disable the tx_power calibration.
In 5000 HW, this calibration is implemented in uCode, hence, it is
disabled in driver by default.

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>