net: ipa: determine replenish doorbell differently

Rather than tracking the number of receive buffer transactions that
have been submitted without a doorbell, just track the total number
of transactions that have been issued.  Then ring the doorbell when
that number modulo the replenish batch size is 0.

The effect is roughly the same, but the new count is slightly more
interesting, and this approach will someday allow the replenish
batch size to be tuned at runtime.

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