Remove checks on MAX_SERIAL_PORTS that are just bounds checks

Remove checks on MAX_SERIAL_PORTS that are just bounds checks

Remove checks on MAX_SERIAL_PORTS that were just checking whether
they were within bounds for the serial_hds[] array and falling
back to NULL if not. This isn't needed with the serial_hd()
function, which returns NULL for all indexes beyond what the
user set up.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-9-peter.maydell@linaro.org