iwlwifi: pcie: fix (again) prepare card flow

The hardware bug in the commit mentioned below forces us
not to re-enable the clock gating in the Host Cluster.
The impact on the power consumption is minimal and it allows
the WAKE_ME interrupt to propagate.

CC: <stable@vger.kernel.org> [4.1+]
Fixes: c9fdec9f

 ("iwlwifi: pcie: fix prepare card flow")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>