Add and use the new helper function adf_get_etr_base() which retrieves
the virtual address of the ring bar.

This will be used extensively when adding support for Live Migration.
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Xin Zeng <xin.zeng@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>