mtd: introduce a macro for max NAND ID sequence length

Introduce a helpful macro for the maximum NAND ID sequence length instead of
using the "8" magic number.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Acked-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>