ACPI: Use EXPORT_SYMBOL() for acpi_bus_get_device()

Commit caf5c03f (ACPI: Move acpi_bus_get_device() from bus.c to
scan.c) caused acpi_bus_get_device() to be exported using
EXPORT_SYMBOL_GPL(), but that broke some binary drivers in
existence, so revert that change.

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>