Add two more comments into the advk_pcie_remove() method.

Link: https://lore.kernel.org/r/20211130172913.9727-6-kabel@kernel.orgSigned-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>