cxl/pci: Rename DOE mailbox handle to doe_mb

cxl/pci: Rename DOE mailbox handle to doe_mb

Trivial variable rename for the DOE mailbox handle from cdat_doe to
doe_mb. The variable name cdat_doe is too ambiguous, use doe_mb that
is commonly used for the mailbox.

Signed-off-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240216155844.406996-2-rrichter@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>