tg3: Add 5784 ASIC rev to earlier PCIe MPS fix

tg3 commit e7126997 entitled
"tg3: Preserve PCIe MPS setting for new devs" attempted to ensure the
PCIe link negotiated Maximum Payload Size (MPS) setting was 128 bytes
for all devices that didn't support higher speeds.  The 5784 device was
mistakenly added to this list when it shouldn't have.  This patch
removes the 5784 ASIC rev devices from that list.

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