staging: vt6656: iwctl.c: Rewrite siwrts funciton

This function has the following issues:
	Parameter info and extra are not used
	Wrong error handling(the function not return -EINVAL when it
happens)

This patch simplifies this funtion, remove the not used parameters and
fix the error handilng.

Signed-off-by: Marcos Paulo de Souza <marcos.mage@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>