thunderbolt: Use correct ICM commands in system suspend

The correct way to put the ICM into suspend state is to send it
NHI_MAILBOX_DRV_UNLOADS mailbox command. NHI_MAILBOX_SAVE_DEVS is not
needed on Intel Titan Ridge so we can skip it.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>