net: dsa: mv88e6xxx: rename smi_mutex to reg_lock

The chip smi_mutex mutex is used to protect the access to the internal
switch registers, not only the Multi-chip Addressing Mode, as commented.

Since we will isolate SMI-specific pieces of code, avoid the confusion
now by renaming smi_mutex to reg_lock. No functional changes here.

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