net: ipa: kill ipa_endpoint_stop()

The previous commit made ipa_endpoint_stop() be a trivial wrapper
around gsi_channel_stop().  Since it no longer does anything
special, just open-code it in the three places it's used.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>