ACPI / SBS: Enable battery manager when present

Commit 9faf6136 (ACPI / SBS: Disable smart battery manager on
Apple) introduced a regression disabling the SBS battery manager.
The battery manager should be marked as present when
acpi_manager_get_info() returns 0.

Fixes: 9faf6136 (ACPI / SBS: Disable smart battery manager on Apple)
Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Cc: 3.18+ <stable@vger.kernel.org> # 3.18+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>