cxl/port: Split endpoint and switch port probe

Jonathan points out that the shared code between the switch and endpoint
case is small. Before adding another is_cxl_endpoint() conditional,
just split the two cases.

Rather than duplicate the "Couldn't enumerate decoders" error message
take the opportunity to improve the error messages in
devm_cxl_enumerate_decoders().

Reported-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/167601999378.1924368.15071142145866277623.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>