mtd: rawnand: gpmi: Fix runtime PM imbalance in gpmi_nand_probe

There is no reason that the failure of __gpmi_enable_clk()
could lead to PM usage counter decrement.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200522101713.24350-1-dinghao.liu@zju.edu.cn