MTD: nand: mxs_nand_spl: Fix empty function pointer for BBT

The initialization function calls a nand_chip.scan_bbt(mtd) but
scan_bbt is never initialized resulting in an undefined function
pointer.  This will direct the function pointer to nand_default_bbt
defined in the same file.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Stefan Agner <stefan.agner@toradex.com>