USB: usbtest.c: length, sglen and vary are unsigned, so cannot be negative

length, sglen and vary are unsigned, so cannot be negative

see
vi drivers/usb/misc/usbtest.c +18
struct usbtest_param {
...
        unsigned                iterations;
        unsigned                length;
        unsigned                vary;
        unsigned                sglen;
...
};

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>