net: ipa: get rid of workaround in ipa_endpoint_stop()

In ipa_endpoint_stop(), a workaround is used for IPA version 3.5.1
where a 1-byte DMA request is issued between GSI channel stop
retries.

It turns out that this workaround is only required for IPA versions
3.1 and 3.2, and we don't support those.  So remove the call to
ipa_endpoint_stop_rx_dma() in that function.  That leaves that
function unused, so get rid of it.

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