tg3: Support 4mb flash sizes for 5717 and 5719

If a 5717 or 5719 NVRAM part is manually strapped and is 2mb in size,
the driver needs to look at the NVRAM size field rather than infer it
from the strapping itself.

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