drivers: serial: uart_async_rx: Fix race condition

There was a wrong order of conditions checking. First indexes were
compared and then completed flag was checked. It was possible that
after checking first condition context is preempted and new data
is written to the buffer and completed flag is set. In that case
both conditions are met but data added in preemption is dropped.
In order to avoid that completed flag must be checked first.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>