backlight: mp3309c: Utilise temporary variable for struct device

We have a temporary variable to keep pointer to struct device.
Utilise it where it makes sense.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Tested-by: Flavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/20240208184313.2224579-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Lee Jones <lee@kernel.org>