net: ipa: decide on doorbell in replenish loop

Decide whether the doorbell should be signaled when committing a
replenish transaction in the main replenish loop, rather than in
ipa_endpoint_replenish_one().  This is a step to facilitate the
next patch.

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