PCI/DOE: Rename DOE protocol to feature

PCI/DOE: Rename DOE protocol to feature

DOE r1.1 replaced all occurrences of "protocol" with the term "feature" or
"Data Object Type".  PCIe r6.1 incorporated that change.

Rename the existing terms protocol with feature.

Link: https://lore.kernel.org/r/20250306075211.1855177-1-alistair@alistair23.me
Signed-off-by: Alistair Francis <alistair@alistair23.me>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>