ashmem: Fix flush routine of ashmem

The ashmem flush function ashmem_flush_cache_range was calling
dmac_clean_range and dmac_flush_range with a user virtual address
which is not allowed. Instead of calling dmac_clean_range or
dmac_flush_range use flush_cache_user_range which can be called
with a user vitual address.

Change-Id: I5dd137fb248b45ef2c7ea5f7bdb4b540b8310af9