milkymist-uart: use qemu_chr_fe_write_all() instead of qemu_chr_fe_write()

milkymist-uart: use qemu_chr_fe_write_all() instead of qemu_chr_fe_write()

qemu_chr_fe_write() is capable of returning 0
to indicate EAGAIN (and friends) and we don't
handle this.

Just change it to qemu_chr_fe_write_all() to fix.

Reported-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>