ahci: remove pmp link online check in FBS EH

commit 912b9ac683b112615d5605686f1dc086402ce9f7 upstream.

ata_link_online() check in ahci_error_intr() is unnecessary, it should
be removed otherwise may lead to lockup with FBS enabled PMP.
http://marc.info/?l=linux-ide&m=137050421603272&w=2

Reported-by: Yu Liu <liuyu.ac@gmail.com>
Signed-off-by: Shane Huang <shane.huang@amd.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>