mtd: rawnand: timings: Avoid redefining tR_max and tCCS_min

These two values are already hardcoded in the default ONFI timing
structure, no need to redefine them here. Plus, we want to be able to
reference timing mode 0 easily and reliably, without extra
computation, so we get rid of the extra assignations.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-17-miquel.raynal@bootlin.com