ahci: Soften up the dmesg on SB600 PMP softreset failure recovery

Too strong words led to spurious bug reports: Novell bugzilla #527748,
RedHat bugzilla #468800. This patch is used to soften up the dmesg on
SB600 PMP softreset failure recovery, so as to remove the scariness and
concern from community.

Reported-by: pgnet Dev <pgnet.dev@gmail.com>
Signed-off-by: Shane Huang <shane.huang@amd.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>