nand: Move the sub-page read support enable to a flag

Use a flag instead of a hard-coded macro so that sub-page reads can be
enabled in other cases (such as on-die ecc).

This is the same as a5ff4f102937a3492bca4a9ff0c341d78813414c in Linux

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>