net: ipa: synchronize NAPI only for suspend

When stopping a channel, gsi_channel_stop() will ensure NAPI
polling is complete when it calls napi_disable().  So there is no
need to call napi_synchronize() in that case.

Move the call to napi_synchronize() out of __gsi_channel_stop()
and into gsi_channel_suspend(), so it's only used where needed.

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