ath79: use gpios for switch management in WZR-HP-G300NH variants

ath79: use gpios for switch management in WZR-HP-G300NH variants

The RTL8366S/RB switch node in DTS defines "mii-bus = <&mdio0>" to permit
management via SMI but this has likely never worked, instead falling back
to using GPIOs in the past:

     rtl8366s switch: cannot find mdio bus from bus handle (yet)
     rtl8366s switch: using GPIO pins 19 (SDA) and 20 (SCK)
     rtl8366s switch: RTL8366 ver. 1 chip found

Recently, the rtl8366s and rtl8366_smi drivers were changed from built-in
to loadable modules. This affected driver probing order and caused switch
initialization (and network access) to fail:

     rtl8366s switch: using MDIO bus 'ag71xx_mdio'
     rtl8366s switch: unknown chip id (ffff)
     rtl8366s switch: chip detection failed, err=-19

Force using GPIOs to manage the switch by dropping the "mii-bus" DTS
definition, which works for both built-in and loadable switch drivers.

Fixes: 6e0f0eae5b ("ath79: use rtl8366s and rtl8366_smi as a module")
Fixes: 575ec7a4b1 ("ath79: use rtl8366rb as a module")
Tested-by: Tony Ambardar <itugrok@yahoo.com> # WZR-HP-G300NH (RTL8366S)
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>