Small bug fix from DragonFly / review requested

[Max Laier]

Joerg Sonnenberger has discovered a problem in the snc(4) driver that might 
result in packets showing up on bpf multiple times.  See the changelog for 
sys/dev/netif/snc/dp83932.c in their source tree[1].

Attached is a fix for HEAD.  I am almost sure that it is right, but I'd 
welcome a review as there might be a chance that sonicput() and the TX 
interrupt free the mbuf before it gets to bpf.  From my reading it seems that 
the mbuf is "safe" until sc->mtd_free is altered (see comment).

Thanks for your input.

[1] http://www.dragonflybsd.org/cvsweb/src/sys/dev/netif/snc/dp83932.c

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