tg3: Remove 40BIT_DMA_LIMIT_BUG

This patch removes the 40BIT_DMA_LIMIT_BUG flag.  There already exists a
flag for this purpose (TG3_FLAG_40BIT_DMA_BUG) and was already being
used in the correct spot.

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>