cxl/pci: Use CXL_MBOX_SUCCESS to check against mbox_cmd return code

Also mention the need for the caller to check against any
errors from the hardware in return_code.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed by: Adam Manzanares <a.manzanares@samsung.com>
Link: https://lore.kernel.org/r/20220404021216.66841-3-dave@stgolabs.net


Signed-off-by: Dan Williams <dan.j.williams@intel.com>