PCI: Add missing MODULE_DESCRIPTION() macros

When ARCH=x86, make allmodconfig && make W=1 C=1 reports:

  WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/pci/pci-stub.o
  WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/pci/pci-pf-stub.o

Add the missing MODULE_DESCRIPTION() macro.

Link: https://lore.kernel.org/r/20240610-md-drivers-pci-v1-1-139c135853ea@quicinc.com


Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
[bhelgaas: update MODULE_DESCRIPTION() text]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>