tg3: Remove 4G_DMA_BNDRY_BUG flag

Now that all chips have this bug, the flag checks become useless code.
This patch removes the flag.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>