mtd: rawnand: mpc5121: Check before clk_disable_unprepare() not needed

All code in clk_disable_unprepare() already checks the clk ptr using
IS_ERR_OR_NULL so there is no need to check it again before calling it.
A lot of other drivers already rely on this behaviour, so it's safe
to do so here.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220512185033.46901-1-phil.edworthy@renesas.com