Fix errors in handling "device label" _DSM return values.

If _DSM returns a Unicode string, the ACPI type is ACPI_TYPE_BUFFER, not
ACPI_TYPE_STRING.  Fix dsm_label_utf16s_to_utf8s() to convert UTF-16 from
acpi_object->buffer instead of acpi_object->string.

Prior to v3.14, we accepted Unicode labels (ACPI_TYPE_BUFFER return
values).  But after 1d0fcef7, we accepted only ASCII (ACPI_TYPE_STRING)
(and we incorrectly tried to convert those ASCII labels from UTF-16 to
UTF-8).

Rejecting Unicode labels made us return -EPERM when reading sysfs
"acpi_index" or "label" files, which in turn caused on-board network
interfaces on a Dell PowerEdge E420 to be renamed (by udev net_id internal)
from eno1/eno2 to enp2s0f0/enp2s0f1.

Fix this by accepting either ACPI_TYPE_STRING (and treating it as ASCII) or
ACPI_TYPE_BUFFER (and converting from UTF-16 to UTF-8).

[bhelgaas: changelog]
Fixes: 1d0fcef7 ("ACPI / PCI: replace open-coded _DSM code with helper functions")
Signed-off-by: Simone Gotti <simone.gotti@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jiang Liu <jiang.liu@linux.intel.com>
CC: stable@vger.kernel.org	# v3.14+