iwlwifi: pcie: allow the op_mode to call stop_device whenever it wants

Calling stop_device when start_fw wasn't called would issue:
Stopping tx queues that aren't allocated...

Also allow the op_mode to call stop_device and then to
disable the Tx queues - in that case just silently ignore
the disabling on the Tx queues, since the PRPH registers
aren't reachable any more.

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