xen/hvc: Collapse error logic.

All of the error paths are doing the same logic. In which
case we might as well collapse them in one path.

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