mtd: rawnand: Hide the chip->data_interface indirection

As a preparation for allocating the data interface structure
dynamically (and rename it), let's avoid accessing
chip->data_interface directly.

Instead, we introduce a helper, nand_get_interface_config(), and use
it to retrieve the current data interface configuration out of a
nand_chip object.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-19-miquel.raynal@bootlin.com