A transaction failure only occurs if no more transactions are
available for an endpoint.  It's a very cheap test.

When replenishing an RX endpoint buffer, there's no point in
allocating pages if transactions are exhausted.  So don't bother
doing so unless the transaction allocation succeeds.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>