mtd: rawnand: orion: Use devm_clk_get_optional()

Use devm_clk_get_optional() instead of hand writing it.

While at it, use dev_err_probe() to further simplify the code. This is also
less verbose if clk_get() returns -EPROBE_DEFER.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/a5bde48e3e1165dd65d1d1c1739e03ace1bef5d3.1659907229.git.christophe.jaillet@wanadoo.fr