char: xillybus: Fix condition for invoking the xillybus/ subdirectory

As Xillybus' configuration symbol hierarchy has been reorganized recently,
the correct condition for compiling the xillybus/ subdirectory is now
CONFIG_XILLYBUS_CLASS, and not CONFIG_XILLYBUS.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20210528092242.51104-1-eli.billauer@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>