nvme: flush dcache on both r/w, and the prp list

It's possible that the data cache for the buffer still holds data
to be flushed to memory, since the buffer was probably used as stack
before.  Thus we need to make sure to flush it also on reads, since
it's possible that the cache is automatically flused to memory after
the NVMe DMA transfer happened, thus overwriting the NVMe transfer's
data.  Also add a missing dcache flush for the prp list.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>