ACPI: dock: fix oops when _DCK evaluation fails

Data returned by acpi_get_name in acpi_buffer is not acpi_object and
therefore should not be cast to it, otherwise we'll get an nice oops
trying to print error message.

Also print name of the ACPI object corresponding to the docking station
and elevate severity of the message printed when _DCK fails to KERN_ERR.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>