mtd: rawnand: Allow SDR timings to be nacked

This should never happen in theory and is probably a controller driver
bug. Anyway it's probably better to bail out at this point if this
happens rather than continuing the boot process.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-18-miquel.raynal@bootlin.com