net: dsa: mv88e6xxx: Fix unused variable warning by using variable

_mv88e6xxx_stats_wait() did not check the return value from
 mv88e6xxx_g1_read(), so the compiler complained about set but unused
 err.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>