net: ipa: move mutex calls into __gsi_channel_stop()

Move the mutex calls out of gsi_channel_stop_retry() and into
__gsi_channel_stop(), to make the latter more semantically similar
to __gsi_channel_start().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>