If multiple NAND chips are registered to the same controller, then when
rebooting the system, the first one will grab the controller lock, while
the second will wait forever for the first one to release it. i.e., a
classic deadlock.

This problem was solved for a similar case (suspend/resume) back in
commit 6b0d9a84 ("mtd: nand: fix multi-chip suspend problem"), and
the shutdown state really isn't much different for us, so rather than
adding a new special case to nand_get_device(), we can just overload the
FL_PM_SUSPENDED state.

Now, multiple chips can "get" the same controller lock (preventing
further I/O), while we still allow other chips to pass through
nand_shutdown().

Original report:
http://thread.gmane.org/gmane.linux.drivers.mtd/59726
http://lists.infradead.org/pipermail/linux-mtd/2015-July/059992.html

Fixes: 72ea4036 ("mtd: nand: added nand_shutdown")
Reported-by: Andrew E. Mileski <andrewm@isoar.ca>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: Andrew E. Mileski <andrewm@isoar.ca>
Acked-by: Scott Branden <sbranden@broadcom.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>