net: ipa: use a Boolean rather than count when replenishing

The count argument to ipa_endpoint_replenish() is only ever 0 or 1,
and always will be (because we always handle each receive buffer in
a single transaction).  Rename the argument to be add_one and change
it to be Boolean.

Update the function description to reflect the current code.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>