cxl/port: Fix port to pci device assumptions in read_cdat_data()

cxl/port: Fix port to pci device assumptions in read_cdat_data()

Not all endpoint CXL ports are associated with PCI devices. The cxl_test
infrastructure models 'struct cxl_port' instances hosted by platform
devices. Teach read_cdat_data() to be careful about non-pci hosted
cxl_memdev instances. Otherwise, cxl_test crashes with this signature:

 RIP: 0010:xas_start+0x6d/0x290
 [..]
 Call Trace:
  <TASK>
  xas_load+0xa/0x50
  xas_find+0x25b/0x2f0
  xa_find+0x118/0x1d0
  pci_find_doe_mailbox+0x51/0xc0
  read_cdat_data+0x45/0x190 [cxl_core]
  cxl_port_probe+0x10a/0x1e0 [cxl_port]
  cxl_bus_probe+0x17/0x50 [cxl_core]

Some other cleanups are included like removing the single-use @uport
variable, and removing the indirection through 'struct cxl_dev_state' to
lookup the device that registered the memdev and may be a pci device.

Fixes: af0a6c3587dc ("cxl/pci: Use CDAT DOE mailbox created by PCI core")
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/168213190748.708404.16215095414060364800.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>