platform/x86: dell-ddv: Improve buffer handling

platform/x86: dell-ddv: Improve buffer handling

When the DDV interface returns a buffer, it actually
returns a acpi buffer containing an integer (buffer size)
and another acpi buffer (buffer content).
The size of the buffer may be smaller than the size of
the buffer content, which is perfectly valid and should not
be treated as an error.
Also use the buffer size instead of the buffer content size
when accessing the buffer to prevent accessing bogus data.

Tested on a Dell Inspiron 3505.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20221102212336.380257-1-W_Armin@gmx.de
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>