mtd: nand: omap: Do not use global variables

Since the commit 97a288ba ("ARM: omap2+: gpmc-nand: Use
dynamic platform_device_alloc()") gpmc-nand driver supports
multiple NAND flash devices connected to the single controller.
Remove global variable to make the code thread-safe.

Signed-off-by: Rostislav Lisovy <lisovy@merica.cz>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>