tty: n_smux: Fix unit test rx retry count race condition

In the smux local loopback unit tests, the get_rx_buff_retry_auto
test fails due to a retry counter being tested prior to waiting
for an smux write completion. This causes the event to be detected
in the next loop, which causes the rx retry watermark to be greater
than the maximum retry, which causes a false unit test failure.

Move counter increment to after the smux write completion.

Change-Id: Iee5d296b4dc38eae5ceb4eb45343b10f00ce37fa
Signed-off-by: Steven Cahail <scahail@codeaurora.org>