net: dsa: mv88e6xxx: Replace stats mutex with SMI mutex

The SMI bus is the bottleneck in all switch operations, not the
granularity of locks. Replace the stats mutex by the SMI mutex to make
the locking concept simpler.

The REG_READ/REG_WRITE macros cannot be used while holding the SMI
mutex, since they try to acquire it. Replace with calls to the
appropriate function which does not try to get the mutex.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>