iflib: allow clone detach if not yet init

If we hit an error during init, then we'll unwind our state and attempt
to detach the device -- don't block it.

This was discovered by creating a wg0 with missing parameters; said
failure ended up leaving this orphaned device in place and ended up
panicking the system upon enumeration of the dev.* sysctl space.

Reviewed by: gallatin, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29145