cxl/port: Reduce number of @dport variables in cxl_port_add_dport()

cxl/port: Reduce number of @dport variables in cxl_port_add_dport()

In preparation for refactoring cxl_port_add_dport() to add RAS register
setup, cleanup the number of dport variables with a dport_exists() helper.

Kill the @dport needed to check for duplicates, rename @new_dport to
@dport.

Reported-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Closes: http://lore.kernel.org/20260116150119.00003bbd@huawei.com
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Terry Bowman <terry.bowman@amd.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20260131000403.2135324-3-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>