iwlwifi: pcie: forbid RTPM on device removal

The pci driver keeps any unbound device in active state and forbids
runtime PM.  When our driver gets probed, we take control of the
state.  When the device is released (i.e. during unbind or module
removal), we should return the state to what it was before.  To do so,
we need to forbid RTPM in the driver remove op.

Additionally, remove an unnecessary pm_runtime_disable() call, move
the initial ref_count setting to a better place and add some comments
explaining what is going on.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>