net: usbnet: support 64bit stats

Add support for the net stats64 counters to the usbnet core. With that
in place put the hooks into every usbnet driver to use it.

This is a strait forward addition of 64bit counters for RX and TX packet
and byte counts. It is done in the same style as for the other net drivers
that support stats64. Note that the other stats fields remain as 32bit
sized values (error counts, etc).

The motivation to add this is that it is not particularly difficult to
get the RX and TX byte counts to wrap on 32bit platforms.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>