mtd: nand: omap: Correct CONFIG_MTD_NAND_OMAP_BCH help message

The MTD_NAND_OMAP_BCH doesn't harm on legacy OMAP platforms
so don't state that it should be disabled for them.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>