mtd: nand: mxs_nand_spl: Add nand_spl_adjust_offset

Since the mxs_nand_spl has implemented adjust read offset in
nand_spl_load_image, so we don't need to check the bad block in
nand_spl_adjust_offset. Directly return the offset to continue
read by nand_spl_load_image.

Signed-off-by: Ye Li <ye.li@nxp.com>