rl(4) TX_UNDERRUN fix

[Joerg Sonnenberger <joerg@xxxxxxxxxxxxxxxxx>]

Hi all,
please test http://leaf.dragonflybsd.org/~joerg/if_rl.c.diff
with any kind of rl(4). This patch is a more aggressive version
of the recently commited patch in FreeBSD 5/6, the version
orginaly proposed in kern/61448.

It shouldn't negatively impact performance or stability.

Time to commit: 1 week

Joerg