Making module name conflicts a fatal error breaks sparc64 allmodconfig:

Error log:
error: the following would cause module name conflict:
  drivers/char/adi.ko
  drivers/input/joystick/adi.ko

Renaming one of the modules would solve the problem, but then cause other
problems because neither of them is automatically loaded and changing
the name is likely to break any setup that relies on manually loading
it by name.

As there is probably no sparc64 system with this kind of ancient joystick
attached, work around it by adding a Kconfig dependency that forbids
them from both being modules.  It is still possible to build the joystick
driver if the sparc64 adi driver is built-in.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200609100643.1245061-1-arnd@arndb.deSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>