xen/swiotlb: For early initialization, return zero on success.

If everything is setup properly we would return -ENOMEM since
rc by default is set to that value. Lets not do that and return
a proper return code.

Note: The reason the early code needs this special treatment
is that it SWIOTLB library call does not return anything (and
had it failed it would call panic()) - but our function does.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>