mtd: nand: fix mention to CONFIG_MTD_NAND_ECC_BCH

Mention to CONFIG_MTD_ECC_BCH in the warning message can be confusing as this
doesn't match the exact name of the configuration option.
This warning showed up once to me when I was starting to set up BCH. After
checking my .config file, it took a moment before realizing it is
CONFIG_MTD_NAND_ECC_BCH instead of CONFIG_MTD_ECC_BCH.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>