arge: ensure there's enough space in the TX ring before attempting to
send frames.

This matches the other check for space.

"enough" is a misnomer, for "reasons". The biggest reason is that
the TX ring is actually a circular linked list, with no head/tail pointers.
This is just a bit more headroom between head/tail so we have time to
schedule frames before we hit where the hardware is at.

Ideally this would be tunable and a little larger.