powerpc/85xx: fix NAND_CMD_READID read bytes number

when nand_get_flash_type() is called, it will read 8 bytes of ID instead of 5,
but the driver only read 5 bytes, so kernel will dump error messages like:

fsl-lbc ffe124000.localbus: read_byte beyond end of buffer
fsl-lbc ffe124000.localbus: read_byte beyond end of buffer
fsl-lbc ffe124000.localbus: read_byte beyond end of buffer

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>