iwlagn: extend notification wait function

A notification wait function is called with the
command, but currently has no way of passing
data back to the caller -- fix that by adding a
void pointer to the function that can be used
between the caller and the function.

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