setserial: Cannot get serial info: Inappropriate ioctl for device

This means that the driver does not support the IOCTL that setserial is using:

setserial gets the information via an ioctl() call. In case the driver 
for your device does not support TIOCGSERIAL, the "invalid argument" is 
returned.

(Debian bug report)

I think stty should be able to perform any configuration you need for a USB-Serial device.