net: ipa: kill replenish_backlog

We no longer use the replenish_backlog atomic variable to decide
when we've got work to do providing receive buffers to hardware.
Basically, we try to keep the hardware as full as possible, all the
time.  We keep supplying buffers until the hardware has no more
space for them.

As a result, we can get rid of the replenish_backlog field and the
atomic operations performed on it.

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