mtd: rawnand: ams-delta: Stop implementing ->select_chip()

Now that the CS to be selected is passed in nand_operation->cs, we can
get rid of the ->select_chip() implementation and replace it by an
internal function which is called from the chip->exec_op() hook.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>