platform/x86: dell-wmi: Add a comment explaining the 0xb2 magic number

The hotkey table is 0xb2, add a comment for clarity.

Suggested-by: Darren Hart <dvhart@infradead.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>