mips: octeon: cvmx-pow.h: fix duplicated words

Delete the repeated words "Returns" and convert to kernel-doc
notation by adding a ':'.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>