Work around netmap issue with ixgbe

After multiple start/stop of netmap, ixgbe will get into a bad state
requiring a reboot to recover. Adding a delay before stopping the interface
appears to work around the issue.

The -CURRENT driver has diverged too far from -STABLE for an MFC.

PR: 221317
Submitted by: Sylvain Galliano <sg@efficientip.com>
Reported by: Cassiano Peixoto <peixoto.cassiano@gmail.com>
Sponsored by: Limelight Networks