[MTD] [NAND] fix subpage read for small page NAND

Current implementation of subpage read feature for NAND has issues with
small page devices. Small page NAND do not support RNDOUT command.
So subpage feature is not applicable for them.

This patch disables support of subpage for small page NAND.
The code is verified on nandsim(SP NAND simulation) and on LP NAND
devices.

Thanks a lot to Artem for finding this issue.

Signed-off-by: Alexey Korolev <akorolev@infradead.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>