Use DEVICE_ATTR*() to simplify definition of the SMBIOS label attributes.
No functional change intended.

Note that dev_attr_smbios_label requires __ATTR() because the "label"
attribute can be exposed via either ACPI or SMBIOS, and we already have the
ACPI label_show() function in this file.

[bhelgaas: split to separate patch]
Link: https://lore.kernel.org/r/20210416205856.3234481-6-kw@linux.comSigned-off-by: NKrzysztof Wilczyński <kw@linux.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>