drm/udl: Make udl driver depend on CONFIG_USB

The udl driver for DisplayLink devices depends on support for host-side
USB controllers, which is enabled with CONFIG_USB. Plain USB support as
given by CONFIG_USB_SUPPORT is not sufficient.

This patch changes dependencies for udl to depend on CONFIG_USB, instead
of CONFIG_USB_SUPPORT. Users will have to enable CONFIG_USB and select a
USB host controller. With this change udl dependencies work the same way
as dependencies for PCI drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106141016.9562-1-tzimmermann@suse.de