The default nand_read_subpage implementation returns -ENOSUPP, so we have
to make sure that barebox never uses this implementation. By setting the
NAND_SUBPAGE_READ flag to 0x0 we ensure NAND_HAS_SUBPAGE_READ() never returns
true and also give the compiler a chance to optimize away some unused code.

subpage reads are a rather exotic feature even in Linux. It is only used on
largepage NANDs with soft ecc. Even if we have this case it needs non page
aligned reads to actually profit from this feature.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Hattink, Tjalling [FINT] <T.Hattink@fugro.nl>