mtd: gpmi: fix a dereferencing freed memory error

The patch "490e280a mtd: gpmi-nand: Convert to module_platform_driver()"
introduced a "dereferencing freed memory" error.

This patch fixes it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>