net: ipa: don't retry in ipa_endpoint_stop()

The only reason ipa_endpoint_stop() had a retry loop was that the
just-removed workaround required an IPA DMA command to occur between
attempts.  The gsi_channel_stop() call that implements the stop does
its own retry loop, to cover a channel's transition from started to
stop-in-progress to stopped state.

Get rid of the unnecessary retry loop in ipa_endpoint_stop().

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