mtd: nand: sh_flctl: Use of_device_get_match_data() helper

Use the of_device_get_match_data() helper instead of open coding.
While at it, make config const so the cast can be dropped.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>