usb: fsl-mph-dr-of: Remove unnecessary NULL check before clk_disable_unprepare()

clk_disable_unprepare() already checks NULL by using IS_ERR_OR_NULL.
Remove unneeded NULL check for clk here.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Link: https://lore.kernel.org/r/20250617041917.1930885-1-nichen@iscas.ac.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>