USB: serial: cp210x: fix control-characters error handling

In the unlikely event that setting the software flow-control characters
fails the other flow-control settings should still be updated (just like
all other terminal settings).

Move out the error message printed by the set_chars() helper to make it
more obvious that this is intentional.

Fixes: 7748feff ("USB: serial: cp210x: add support for software flow control")
Cc: stable@vger.kernel.org	# 5.11
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>