iwlwifi: configure the LTR

The LTR is the handshake between the device and the root
complex about the latency allowed when the bus exits power
save. This configuration was missing and this led to high
latency in the link power up. The end user could experience
high latency in the network because of this.

Cc: <stable@vger.kernel.org> [3.10+]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>