ACPI: bus: Introduce acpi_dev_get() and reuse it in ACPI code

Introduce acpi_dev_get() to have a symmetrical API with acpi_dev_put()
and reuse both in ACPI code in drivers/acpi/.

While at it, use acpi_bus_put_acpi_device() in one place instead of
the above.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>