IPA v4.2 has a hardware quirk that requires the AP to allocate GSI
channels for the modem to use.  It is recommended that these modem
channels get stopped (with a HALT generic command) by the AP when
its IPA driver gets removed.

The AP has no way of knowing the current state of a modem channel.
So when the IPA driver issues a HALT command it's possible the
channel is not running, and in that case we get an error indication.
This error simply means we didn't need to stop the channel, so we
can ignore it.

This patch adds an explanation for this situation, and arranges for
this condition to *not* report an error message.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>