Allwinner SoC's PHY 0, when used as OTG controller, have no pmu part.
The code that poke some unknown bit of PMU for H3/A64 didn't check
the PHY, and will cause kernel oops when PHY 0 is used.

This patch will check whether the pmu is not NULL before poking.

Fixes: b3e0d141 (phy: sun4i: add support for A64 usb phy)
Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>