mtd: rawnand: fsmc: Keep bank enable bit set

Hammering the "bank enable" (PBKEN) bit on and off between
every command crashes the Nomadik NHK15 with this message:

Scanning device for bad blocks
Unhandled fault: external abort on non-linefetch (0x008) at 0xcc95e000
pgd = (ptrval)
[cc95e000] *pgd=0b808811, *pte=40000653, *ppte=40000552
Internal error: : 8 [#1] PREEMPT ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.20.0-rc2+ #72
Hardware name: Nomadik STn8815
PC is at fsmc_exec_op+0x194/0x204
(...)

After a discussion we (me and Boris Brezillon) start to suspect
that this bit does not immediately control the chip select line
at all, it rather enables access to the bank and the hardware
will drive the CS autonomously. If there is a NAND chip connected,
we should keep this enabled.

As fsmc_nand_setup() sets this bit, we can simply remove the
offending code.

Fixes: 550b9fc4 ("mtd: rawnand: fsmc: Stop implementing ->select_chip()")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>