mtd: rawnand: vf610: Stop implementing ->select_chip()

Now that the CS to be selected is kept in chip->cur_cs and 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() and chip->ecc.read/write_xxx() hooks.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>