mtd: nand: xway: disable module support

The xway_nand driver accesses the ltq_ebu_membase symbol which is not
exported. This also should not get exported and we should handle the
EBU interface in a better way later. This quick fix just deactivated
support for building as module.

Fixes: 99f2b107 ("mtd: lantiq: Add NAND support on Lantiq XWAY SoC.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>