media: atomisp: move null check to earlier point

`find_gmin_subdev()` that returns a pointer to `struct
gmin_subdev` can return NULL.

In `gmin_v2p8_ctrl()` there's a call to this function but the
possibility of a NULL was not checked before its being dereferenced,
i.e.:

  /* Acquired here --------v */
  struct gmin_subdev *gs = find_gmin_subdev(subdev);

  /*  v------Dereferenced here */
  if (gs->v2p8_gpio >= 0) {
      ...
  }

With this change we're null checking `find_gmin_subdev()` result
and we return an error if that's the case. We also WARN()
for the sake of debugging.

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
Reported-by: Coverity Static Analyzer CID 1465536
Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>