mtd: nand: denali_dt: replace clk_disable() with clk_disable_unprepare()

The denali_dt_probe() calls clk_disable_unprepare() in the bailout
path, whereas denali_dt_remove calls clk_disable(), inconsistently.
Replace the latter with clk_disable_unprepare() to make sure to
unprepare the clock.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>