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

Now that the CS line to assert is directly passed through the
nand_operation struct we can replace the fsmc_select_chip()
implementation by an internal fsmc_ce_ctrl() function which is
directly called from fsmc_exec_op()

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