xen/hvc: Collapse error logic.

commit 2e5ad6b9c45d43cc4e7b8ac5ded1c55a7c4a3893 upstream.

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

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>