iwlwifi: provide proper API to disable all interrupts

Since the op_mode may go away, the transport needs to be able to
be told not to update the op_mode at all (even for RF kill).
Provide this API and use it in the proper places.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>