MIPS: smp-cps: Fix entry code cache flush for systems with coherent I/O

MIPS: smp-cps: Fix entry code cache flush for systems with coherent I/O

The dma_cache_wback_inv function performs exactly as is required here,
unless the system has coherent I/O in which case it's a no-op. Call the
underlying cache writeback functions directly, which is arguably clearer
anyway given that the code doesn't actually have anything to do with
DMA in a strict sense.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7282/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>