PCI: pci-bridge-emul: Add description for class_revision field

PCI: pci-bridge-emul: Add description for class_revision field

commit 9319230ac147067652b58fe849ffe0ceec098665 upstream.

The current assignment to the class_revision member

  class_revision |= cpu_to_le32(PCI_CLASS_BRIDGE_PCI << 16);

can make the reader think that class is at high 16 bits of the member and
revision at low 16 bits.

In reality, class is at high 24 bits, but the class for PCI Bridge Normal
Decode is PCI_CLASS_BRIDGE_PCI << 8.

Change the assignment and add a comment to make this clearer.

Link: https://lore.kernel.org/r/20211130172913.9727-2-kabel@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>