ACPI / dock: Use acpi_device_enumerated() to check if dock is present

After commit 202317a5 (ACPI / scan: Add acpi_device objects for
all device nodes in the namespace) acpi_bus_get_device() will always
return 0 for dock devices in dock_notify(), so the dock station
docking code under ACPI_NOTIFY_DEVICE_CHECK will never be executed
and docking will not work as a result of that.

Fix the problem by making dock_notify() use acpi_device_enumerated()
to check the presence of the device instead of checking the return
value of acpi_bus_get_device().

Fixes: 202317a5

 (ACPI / scan: Add acpi_device objects for all device nodes in the namespace)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>