net: dsa: mv88e6xxx: Split mv88e6xxx_reg_read and mv88e6xxx_reg_write

Split mv88e6xxx_reg_read and mv88e6xxx_reg_write into two functions each,
one to acquire smi_mutex and one to get struct mii_bus *bus from
struct dsa_switch *ds and to call the actual read/write function.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>