hw/char/xilinx_uartlite: Replace hw_error() by qemu_log_mask()

hw_error() calls exit(). This a bit overkill when we can log
the accesses as unimplemented or guest error.

When fuzzing the devices, we don't want the whole process to
exit. Replace some hw_error() calls by qemu_log_mask().

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200518140309.5220-4-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>