tg3: restore rx_dropped accounting

commit 511d2224 (tg3: 64 bit stats on all arches), overlooked the
rx_dropped accounting.

We use a full "struct rtnl_link_stats64" to hold rx_dropped value, but
forgot to report it in tg3_get_stats64().

Use an "unsigned long" instead to shrink "struct tg3" by 176 bytes, and
report this value to stats readers.

Increment rx_dropped counter for oversized frames.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Michael Chan <mchan@broadcom.com>
CC: Matt Carlson <mcarlson@broadcom.com>
Acked-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>