[TG3]: Fix bug in 40-bit DMA workaround code

Need to check the TG3_FLAG_40BIT_DMA_BUG flag in the workaround code
path instead of device flags.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tg3.c b/drivers/net/tg3.c
index c41dbb0..0fafc52 100644
--- a/drivers/net/tg3.c
+++ b/drivers/net/tg3.c
@@ -3600,7 +3600,7 @@
 					  int len)
 {
 #if defined(CONFIG_HIGHMEM) && (BITS_PER_LONG == 64)
-	if (tp->tg3_flags2 & TG3_FLG2_5780_CLASS)
+	if (tp->tg3_flags & TG3_FLAG_40BIT_DMA_BUG)
 		return (((u64) mapping + len) > DMA_40BIT_MASK);
 	return 0;
 #else