phy: sun4i-usb: change PHYCTL register clearing code

It seems that all SoCs with the PHYCTL register offset as 0x10 need the
PHYCTL register to be cleared before it's written.

Change PHYCTL register clearing code to judge whether clearing is needed
based on the PHYCTL offset.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>