mtd: sharpslpart: make local function sharpsl_nand_cleanup_ftl() static

Fixes the following sparse warnings:

drivers/mtd/parsers/sharpslpart.c:222:6: warning:
 symbol 'sharpsl_nand_cleanup_ftl' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>