mtd: gpmi-nand: fix error return from gpmi_get_clks()

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>