phy: sun4i: check PMU presence when poking unknown bit of pmu

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>