net: dsa: mv88e6xxx: assert SMI lock

It's easy to forget to lock the smi_mutex before calling the low-level
_mv88e6xxx_reg_{read,write}, so add a assert_smi_lock function in them.

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