tg3: Fix NVRAM page writes on newer devices

On newer devices, the hardware expects the NVRAM address register
to be written only once per NVRAM page.  To do otherwise causes NVRAM
corruption.  This patch fixes the problem.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>