phy: mediatek: Fix Kconfig indentation

Adjust indentation from spaces to tab (+optional two spaces) as in
coding style.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>