serial: st-asc: Fix data corruption during long console burstsbugs/st-asc-avalanche

On my test platform (B2020/STiH416) the serial port issues bad characters
during the initial message avalanche as the console comes up. The problem
also occurs when dense(ish) I/O is done using the polled I/O interface.

The problem is fixed for me by using the FIFO half-empty bit rather than
FIFO full bit. Note that using the half-empty bit causes the FIFO to be
managed in a similar way to interrupt based I/O (i.e. where the hardware
gets best test coverage).

Running the FIFO half full will have no impact (good or bad) on console
performance. The UART will still remain fully saturated and the busy-wait
until the FIFO is empty in asc_console_write() will complete at the same
time.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

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