iwlwifi: abstract out notification wait support

This will be sharable, but needs to live in the
op_mode as it is dependent on command processing.
Make a library out of the notification wait code.

Since I wrote all of the code originally and only
Intel employees changed it, we can also relicense
it to dual BSD/GPL.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>