mtd: nand: pxa3xx: Remove hardcoded mtd name

There's no advantage in using a hardcoded name for the mtd device.
Instead use the provided by the platform_device.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>