ppc: Fix roll over bug in flush_cache()

If we call flush_cache(0xfffff000, 0x1000) it would never
terminate the loop since end = 0xffffffff and we'd roll over
our counter from 0xfffffe0 to 0 (assuming a 32-byte cache line)

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>