acer-wmi: Fix capitalisation of GUID

6AF4F258-B401-42fd-BE91-3D4AC2D7C0D3 needs to be
6AF4F258-B401-42FD-BE91-3D4AC2D7C0D3 to match the hardware alias.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: stable@kernel.org