net: dsa: mv88e6xxx: Fix deadlock by double lock

ethtool -S on a DSA interface can deadlock for some switches because
the same lock is taken twice. Use the register read function which
expects the lock to be already held.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 31888234 ("net: dsa: mv88e6xxx: Replace stats mutex with SMI mutex")
Signed-off-by: David S. Miller <davem@davemloft.net>