mmc: atmel-mci: Replace platform device pointer by generic one

There no need to keep a pointer to a platform device as it's not
used outside of ->probe() and ->remove() callbacks.

Replace platform device pointer by generic one in host structure.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240417165708.2965612-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>