mtd: remove superfluous name casts

map_info.name is "const char *"

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>