[ Upstream commit ebc37af5e0a134355ea2b62ed4141458bdbd5389 ]

The device_for_each_child() doesn't require the returned value to be checked.
Thus, drop the dummy variable completely and have no warning anymore:

drivers/spi/spi.c: In function ‘spi_unregister_controller’:
drivers/spi/spi.c:2480:6: warning: variable ‘dummy’ set but not used [-Wunused-but-set-variable]
  int dummy;
      ^~~~~
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>