mtd: nand: fsmc: create and use mtd_to_fsmc()

Create and use mtd_to_fsmc() to avoid duplication of
container_of(mtd, struct fsmc_nand_data, mtd) calls.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>