dell-rbtn: Add a comment about the XPS 13 9350

On the XPS 13 9350, the dell-rbtn mechanism has a new device id, and
the DSDT turns it off if a new enough _OSI is supported.  Add a
comment about why we don't bother supporting it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>