iwlwifi: pcie: don't update the op_mode if rfkill hasn't changed

This is useless and introduces a dependency between rfkill
and stop_device - the op_mode can't call stop_device from
the rfkill notification since it would lead to an endless
recursion.
Next patches will need to do so.

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