media: stm32-csi: Fix dereference before NULL check

commit 80eaf326 upstream.

In 'stm32_csi_start', 'csidev->s_subdev' is dereferenced directly while
assigning a value to the 'src_pad'. However the same value is being
checked against NULL at a later point of time indicating that there
are chances that the value can be NULL.

Move the dereference after the NULL check.

Fixes: e7bad98c ("media: v4l: Convert the users of v4l2_get_link_freq to call it on a pad")
Cc: stable@vger.kernel.org
Signed-off-by: Chandra Mohan Sundar <chandramohan.explore@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>