nand: Don't use qdev_create() in nand_init()

Commit 7426aa72 (nand: Don't inherit
from Sysbus) changed the parent type of TYPE_NAND but continued to use
qdev_create(), which handled a NULL BusState as SysBus.

Use object_new() instead, and reuse the TYPE_NAND define while at it.

Reported-by: Markus Armbruster <armbru@redhat.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>