mtd: devices: elm: clean elm_load_syndrome

This patch refactors elm_load_syndrome() to make it scalable for newer
ECC schemes by removing scheme specific macros (like ECC_BYTES*xx),
and instead using ECC control information passed during elm_config.

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>