Update the comment about what happens when link goes down after we have
checked for link-up. If a PIO request is done while link-down, we have
a serious problem.

Link: https://lore.kernel.org/r/20220110015018.26359-23-kabel@kernel.orgSigned-off-by: NMarek Behún <kabel@kernel.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>