[POWERPC] Remove bogus comment in dma_direct_alloc_coherent()

Since commit c80d9133

 (Make direct DMA use
node local allocations) went in this comment makes no sense.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>