tg3: Apply short DMA frag workaround to 5906

5906 devices also need the short DMA fragment workaround.  This patch
makes the necessary change.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: David S. Miller <davem@davemloft.net>