ACPI: bus: Eliminate acpi_bus_get_device()

Replace the last instance of acpi_bus_get_device(), added recently
by commit 87e59b36 ("spi: Support selection of the index of the
ACPI Spi Resource before alloc"), with acpi_fetch_acpi_dev() and
finally drop acpi_bus_get_device() that has no more users.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mark Brown <broonie@kernel.org>