iwlwifi: move notification wait into core

This code is a library to be used by multiple
opmodes, so move it into the iwlwifi module.

Reviewed-by: Donald H Fry <donald.h.fry@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>