[PATCH v2] staging: atomisp: move null check to earlier point

[Cengiz Can <cengiz@kernel.wtf>]

`find_gmin_subdev` function 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. ie:

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

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

To avoid the issue, null check has been moved to an earlier point
and return semantics has been changed to reflect this exception.

Please do note that this change introduces a new return value to
`gmin_v2p8_ctrl`.

[NEW] - raise a WARN and return -ENODEV if there are no subdevices.
      - return result of `gpio_request` or `gpio_direction_output`.
      - return 0 if GPIO is ON.
      - return results of `regulator_enable` or `regulator_disable`.
      - according to PMIC type, return result of `axp_regulator_set`
        or `gmin_i2c_write`.
      - return -EINVAL if unknown PMIC type.

Caught-by: Coverity Static Analyzer CID 1465536
Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
---
 drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c b/drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c
index 0df46a1af5f0..1ad0246764a6 100644
--- a/drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c
+++ b/drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c
@@ -871,6 +871,9 @@ static int gmin_v2p8_ctrl(struct v4l2_subdev *subdev, int on)
 	int ret;
 	int value;
 
+	if (WARN_ON(!gs))
+		return -ENODEV;
+
 	if (gs->v2p8_gpio >= 0) {
 		pr_info("atomisp_gmin_platform: 2.8v power on GPIO %d\n",
 			gs->v2p8_gpio);
@@ -881,7 +884,7 @@ static int gmin_v2p8_ctrl(struct v4l2_subdev *subdev, int on)
 			pr_err("V2P8 GPIO initialization failed\n");
 	}
 
-	if (!gs || gs->v2p8_on == on)
+	if (gs->v2p8_on == on)
 		return 0;
 	gs->v2p8_on = on;
 
-- 
2.27.0