spl: nand: simple: replace readb() with chip specific read_buf()

Some NAND controllers define custom functions to read data out,
respect this in order to correctly support bad block handling in
simple SPL NAND framework.

NAND controller specific read_buf() is used even to read 1 byte in
case of connected 8-bit NAND device, it turns out that read_byte()
may become outdated.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tom Warren <twarren@nvidia.com>
Acked-by: Scott Wood <scottwood@freescale.com>