libbinder: Flush excess refs after single async transaction

After processing a single async transaction, RpcState previously
returned without flushing the excess Binder references. This
fixes the control flow so that the refs get flushed for any
number of transactions.

Bug: 259517277
Test: trusty_stats_test
Change-Id: I2665f3b0d6120d4d79f46ef5141c9ff5f85fd5b5