iwlwifi: increase DEFAULT_MAX_TX_POWER

The chip is able to transmit up to 22dBm, so set
the constant appropriately.

CC: <stable@vger.kernel.org> [3.13+]
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>