media: atomisp: Use temporary variable for device in gmin_subdev_add()

Use temporary variable for device in gmin_subdev_add().

While here, drop unused temporary variable for device in other places.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>