mmc: atmel-mci: Use temporary variable for struct device

Use temporary variable for struct device to make code neater.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240417165708.2965612-3-andriy.shevchenko@linux.intel.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>