libata: report link resume failure as KERN_WARNING instead of ERR

Link resume failure in itself isn't an error condition and may happen
regularly depending on hardware configuration.  Reporting it as
KERN_ERR makes the condition unnecessarily prominent (e.g. reported
during boot).  Use KERN_WARNING instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: David Shaw <dshaw@jabberwocky.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>