net: dsa: mv88e6xxx: call _mv88e6xxx_stats_wait with SMI lock held

At switch setup, _mv88e6xxx_stats_wait was called without holding the
SMI mutex. Fix this by requesting the lock for this call.

Also, return the _mv88e6xxx_stats_wait code, since it may fail.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>