rules: load drivers only on "add" events

Previously we were loading kernel modules on all device events save
for "remove". With the introduction of KOBJ_BIND/KOBJ_UNBIND this causes
issues, as driver modules that have devices bound to their drivers get
immediately reloaded, and it appears to the user that module unloading
does not work.

Let's change the rules to only load modules on "add" events instead.