ifmedia_add() allocates an ifmedia_entry during ena_attach.
Current code doesn't release this memory during ena_detach()
This commit calls ifmedia_removeall() to properly free the
allocated memory during ena_detach().
This bug was first described in:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278100
Approved by: cperciva
MFC after: 2 weeks
Sponsored by: Amazon, Inc.