cxl: Don't ignore add_process_element() result when attaching context

Currently when attaching a context in dedicated mode, we ignore the
result of add_process_element(), which could potentially fail.

If add_process_element() returns an error, pass it back to the caller.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>