[media] Input: synaptics-rmi4: disallow impossible configuration

The newly added debug mode for the synaptics-rmi4 driver relies on
the v4l2 interface and vb2_vmalloc, but those might be configured
as loadable modules when the driver itself is built-in, resulting
in a link failure:

drivers/input/rmi4/rmi_core.o: In function `rmi_f54_remove':
rmi_f54.c:(.text.rmi_f54_remove+0x14): undefined reference to `video_unregister_device'
rmi_f54.c:(.text.rmi_f54_remove+0x20): undefined reference to `v4l2_device_unregister'
drivers/input/rmi4/rmi_core.o: In function `rmi_f54_vidioc_s_input':
rmi_f54.c:(.text.rmi_f54_vidioc_s_input+0x10): undefined reference to `video_devdata'
drivers/input/rmi4/rmi_core.o: In function `rmi_f54_vidioc_g_input':
rmi_f54.c:(.text.rmi_f54_vidioc_g_input+0x10): undefined reference to `video_devdata'
drivers/input/rmi4/rmi_core.o: In function `rmi_f54_vidioc_fmt':
rmi_f54.c:(.text.rmi_f54_vidioc_fmt+0x10): undefined reference to `video_devdata'
drivers/input/rmi4/rmi_core.o: In function `rmi_f54_vidioc_enum_input':
rmi_f54.c:(.text.rmi_f54_vidioc_enum_input+0x10): undefined reference to `video_devdata'
drivers/input/rmi4/rmi_core.o: In function `rmi_f54_vidioc_querycap':
...

The best workaround I could come up with is to disallow the debug
mode unless it's actually possible to call it.

Fixes: 3a762dbd ("[media] Input: synaptics-rmi4 - add support for F54 diagnostics")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>