mtd: devices: elm: update DRIVER_NAME as "omap-elm"

use "omap-elm" as DRIVER_NAME

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>