usb.h: Always declare usb function prototypes

There is no harm in declaring the function prototypes even if nothing
implements them, and when CONFIG_DM_USB=y the various usb functions are
available regardless of any controller drivers being enabled.

This fixes compile warnings due to missing prototypes on ARCHs where
the ARCH Kconfig always enables CONFIG_DM_USB and various usb drivers.

One could argue that in the case of no controllers CONFIG_DM_USB should not
be set, but this problem is typically seen during bringup of boards which
do actually have usb controllers.

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