thunderbolt: Populate PG field in hot plug acknowledgment packet

USB4 1.0 section 6.4.2.7 specifies a new field (PG) in notification
packet that is sent as response of hot plug/unplug events. This field
tells whether the acknowledgment is for plug or unplug event. This needs
to be set accordingly in order the router to send further hot plug
notifications.

To make it simpler we fill the field unconditionally. Legacy devices do
not look at this field so there should be no problems with them.

While there rename tb_cfg_error() to tb_cfg_ack_plug() and update the
log message accordingly. The function is only used to ack plug/unplug
events.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20191217123345.31850-4-mika.westerberg@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>