mtd: nand: vf610: Check the return value from clk_prepare_enable()

clk_prepare_enable() may fail, so we should better check its return value
and propagate it in the case of error.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>