media: rockchip: rkisp1: use device name for debugfs subdir name

While testing Rockchip RK3399 with both ISPs enabled, a dmesg error
was observed:
```
[   15.559141] debugfs: Directory 'rkisp1' with parent '/' already present!
```

Fix it by using the device name for the debugfs subdirectory name
instead of the driver name, thus preventing name collision.

Link: https://lore.kernel.org/linux-media/20211010175457.438627-1-mike.rudenko@gmail.com


Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>