net: dsa: mv88e6xxx: rename mv88e6xxx_ops

The mv88e6xxx_ops is used to describe how to access the chip registers.
It can be through SMI (via an MDIO bus), or via another interface such
as crafted remote management frames.

The correct BUS operations structure is chosen at runtime, depending on
the chip address and connectivity.

We will need the mv88e6xxx_ops name for future chip-wide operation
structure, thus rename mv88e6xxx_ops to more explicit mv88e6xxx_bus_ops.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>