of: unittest: make unittest_gpio_remove() consistent with unittest_gpio_probe()

On the ->remove() stage the callback uses physical device node instead of one
from GPIO chip and the variable name which is different to one used in
unittest_gpio_probe(). Make these consistent with unittest_gpio_probe().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220708214539.7254-2-andriy.shevchenko@linux.intel.com