usb: usbtest: Remove stack VLA usage

The kernel would like to have all stack VLA usage removed[1].  We
already have a pre-processor constant defined MAX_SGLEN.  We can use
this instead of the variable param-sglen.

[1]: https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>