rge: use C style comments instead of C++

[chris.longros_gmail.com]

Summary

FreeBSD style(9) mandates C style comments. The initial import from
OpenBSD left several C++ style // comments in if_rge.c and if_rgevar.h.
Replace them with proper /* */ comments.

Also fix a malformed comment that mixed // with a closing */.

Signed-off-by: Christos Longros <[e-mail]>