ipack/devices/ipoctal: rework disable TX when the TX buffer is empty

Depending of the device, it disables the TX mode in different places when there
is no more data to transmit.

This patch reorder them and disable the TX mode in the same place.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>