[MTD] [NAND] Correct misspelled preprocessor variable.

Replace the apparently misspelled preprocessor variable
"MTD_NAND_DISKONCHIP_BBTWRITE" with the correct form
"CONFIG_MTD_NAND_DISKONCHIP_BBTWRITE".

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>