mtd: nand: fsl-ifc: fix support of multiple NAND devices

Currently the chipselect used to identify the corresponding NAND chip
is stored at the controller and only set during fsl_ifc_chip_init().
This way, only the last NAND chip is working, as the previous value
of cs_nand gets overwritten.

In order to solve this issue the chipselect is computed on demand by
evaluating the bank variable. Thus, the correct chipselect for each
NAND chip operation is used.

Tested on hardware with two NAND chips connected to the IFC
controller.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Scott Wood <oss@buserror.net>
[YS: reformatted commit message]
Reviewed-by: York Sun <york.sun@nxp.com>