thunderbolt: icm: Ignore mailbox errors in icm_suspend()

On one of my test machines nhi_mailbox_cmd() called from icm_suspend()
times out and returnes an error which then is propagated to the
caller and causes the entire system suspend to be aborted which isn't
very useful.

Instead of aborting system suspend, print the error into the log
and continue.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Michael Jamet <michael.jamet@intel.com>