sdhci: acpi: Use new method to get ACPI companion

ACPI_COMPANION() macro reduces a code to get a companion device out of
struct device.

Use it instead of an old method.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>