net: dsa: b53: Fix B53_SERDES_DIGITAL_CONTROL offset

Maths went wrong, to get 0x20, we need to do 0x1e + (x) * 2, not 0x18,
fix that offset so we access the correct registers. This would make us
not access the correct SerDes Digital control words, status would be
fine and so we would not be correctly flipping between Fiber and SGMII
modes resulting in incorrect status words being pulled into the SerDes
digital status register.

Fixes: 0e01491d ("net: dsa: b53: Add SerDes support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>