staging: xillybus: force to be a module

As reported, this driver causes problems when built into the kernel, so
force it to be a module until those issues are fixed.

Also delete the "default n" lines in the Kconfig, as they are not
needed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>