mtd: rawnand: ams-delta: Don't hardcode read/write pulse widths

Instead of forcing Amstrad Delta specific read/write pulse widths, use
variables initialised from respective fields of chip SDR timings.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-8-jmkrzyszt@gmail.com