dsa: fix 88e6xxx statistics counter snapshotting

The bit that tells us whether a statistics counter snapshot operation
has completed is located in the GLOBAL register block, not in the
GLOBAL2 register block, so fix up mv88e6xxx_stats_wait() to poll the
right register address.

Signed-off-by: Stephane Contri <Stephane.Contri@grassvalley.com>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>