serial: serial-uclass: Use force parameter in stdio_deregister_dev()

On my x86 platform I've noticed, that calling dm_uninit() or the new
function dm_remove_devices_flags() does not remove the desired device at
all. Debugging showed, that the serial uclass returns -EPERM in
serial_pre_remove(). This patch sets the force parameter when calling
stdio_deregister_dev() resulting in a removal of the device.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>