ARM: imx: Fix typo 'MACH_MX31_3DS_MXC_NAND_USE_BBT'

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>