tg3: Avoid panic from reserved statblk field access

tg3: Avoid panic from reserved statblk field access

When RSS is enabled, interrupt vector 0 does not receive any rx traffic.
The rx producer index fields for vector 0's status block should be
considered reserved in this case.  This patch changes the code to
respect these reserved fields, which avoids a kernel panic when these
fields take on non-zero values.

(cherry picked from commit f891ea1)
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Joe Jin <joe.jin@oracle.com>