cxl: Rework error message for incompatible slots

Improve the error message shown if a capi adapter is plugged on a
capi-incompatible slot directly under the PHB (no intermediate switch).

Fixes: 56328743

 ("cxl: Add support for POWER9 DD2")
Cc: stable@vger.kernel.org # 4.14+
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200407115601.25453-1-fbarrat@linux.ibm.com