net: dsa/mv88e6xxx: add reg read and write debug

This commit adds debug messages for the generic mv88e6xxx read and write
routines. The output is similar to this:

    mdio-gpio mdio-gpio.0: <- addr: 0x1b reg: 0x05 val: 0x4000
    mdio-gpio mdio-gpio.0: -> addr: 0x1b reg: 0x07 val: 0x3113
    mdio-gpio mdio-gpio.0: -> addr: 0x1b reg: 0x08 val: 0x0330
    mdio-gpio mdio-gpio.0: -> addr: 0x1b reg: 0x09 val: 0x0000

This is convenient to dynamically debug operations through debugfs with:

    echo file mv88e6xxx.c +p > <debugfs>/dynamic_debug/control

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