tg3: fix 64 bit counter for ethtool stats

Ethtool stats are 64-bits in length.  net_device_stats members are
unsigned long types.  When gathering information for
a get_ethtool_stats call, the driver will call a driver-private,
inlined get_stat64() function, which returns an unsigned long value.
This call will inadvertently mask off the upper 32-bits of a stat on
32-bit machines.

This patch defines a new get_estat() inline function and modifies the
ESTAT_ADD() macro to use it.

Signed-off-by: Stefan Buehler <stbuehler@web.de>
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>