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

Make sure that the driver crtscts state is not updated in the unlikely
event that the flow-control request fails. Not doing so could break RTS
control.

Fixes: 5951b850 ("USB: serial: cp210x: suppress modem-control errors")
Cc: stable@vger.kernel.org	# 5.11
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>