thinkpad_acpi: Remove ambiguous logging for "Unsupported brightness interface"

"Unsupported brightness interface" message gets logged on
 machines that are well supported.

Signed-off-by: Eric Curtin <ericcurtin17@gmail.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>