mmc: slot-gpio: Update default label when no con_id provided

Currently default label of GPIO is assigned to the device name,
when no con_id provided. Instead, let's update it to reflect
what it's about (use already prepared template).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210929111757.52625-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>