[TG3]: Improve NVRAM sizing.

This patch changes the NVRAM sizing procedure so that the driver can
take advantage of devices with 1:1 NVRAM strapping configurations.  This
is useful in cases where the traditional NVRAM sizing method fails.  In
the event that the flash size cannot be determined, the largest known
NVRAM size is used.  The patch also removes support for 5755 NVRAM
devices that are not supported by Broadcom and adds explicit sizing for
this device.

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>