via-velocity: unconditionally drop frames with bad l2 length

By default the driver allowed incorrect frames to be received. What is
worse the code does not handle very short frames correctly. The FCS
length is unconditionally subtracted, and the underflow can cause
skb_put to be called with large number after implicit cast to unsigned.
And indeed, an skb_over_panic() was observed with via-velocity.

This removes the module parameter as it does not work in it's
current state, and should be implemented via NETIF_F_RXALL if needed.

Suggested-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>