mtd: rawnand: Use the data interface mode entry when relevant

The data interface setup does not care about the default timing mode
but cares about the actual timing mode at the time of the call of this
helper.

Use this entry instead and let chip->default_timing_mode only be used
at initialization time.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-9-miquel.raynal@bootlin.com