media: st-delta: Remove unneeded NULL check in delta-v4l2

clk_prepare_enable() already checks NULL by using !clk.
clk_disable_unprepare() already checks NULL by using IS_ERR_OR_NULL.
Remove unneeded NULL checks before them.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>