mtd: fsl-quadspi: Let the clocks error path be clearer

When clk_prepare_enable(q->clk) fails it is clearer to disable the previous
acquired clock (q->clk_en) in the error path rather than doing it locally.

So disable q->clk_en in the error path only.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>