libata: fix last_reset timestamp handling

ehc->last_reset is used to ensure that resets are not issued too
close to each other.  It's initialized to jiffies minus one minute
on EH entry.  However, when new links are initialized after PMP is
probed, new links have zero for this timestamp resulting in long wait
depending on the current jiffies.

This patch makes last_set considered iff ATA_EHI_DID_RESET is set, in
which case last_reset is always initialized.  As an added precaution,
WARN_ON() is added so that warning is printed if last_reset is
in future.

This problem is spotted and debugged by Shane Huang.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Shane Huang <Shane.Huang@amd.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>