usb: Do not log an error when no devices is plugged into a root-hub-less hcd

Before this commit u-boot would print the following on boot with musb and
no usb device plugged in:

starting USB...
USB0:   Port not available.
USB error: all controllers failed lowlevel init

This commit changes this to:

starting USB...
USB0:   Port not available.

Which is the correct thing to do since the low-level init went fine.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>