powerpc/cell: cbe_thermal.c: Cleaning up a variable is of the wrong type

This variable is of the wrong type, everywhere it is used it
should be an unsigned int rather than a int.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>