drm/udl: Replace BUG_ON() with WARN_ON()

BUG_ON() is a tasteless choice as a sanity check for a driver like UDL
that isn't really a core code.  Replace with WARN_ON() and proper
error handling instead.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220804075826.27036-5-tiwai@suse.de