mtd: nand-gpio: Rename internal variables to match functionality

struct platform_device *dev => pdev
struct nand_chip *this => chip

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>