mtd: onenand: unexport onenand_default_bbt()

The onenand_default_bbt() function is only used by the OneNAND core
and not by drivers, so there is no real need to export
it. Additionally, the corresponding nand_default_bbt() for regular
NANDs is not exported either, so for consistency reasons, this commit
removes the EXPORT_SYMBOL on onenand_default_bbt().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>