iwlwifi: pcie: keep the NIC awake when commands are in flight

Under very specific circumstances, the firmware might
ignore a host command. This was debugged and we ended up
seeing that the power management hardware was faulty.
In order to workaround this issue, we keep the NIC awake
as long as we have host commands in flight. This will avoid
to put the hardware into buggy condition.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>