USB: usbtest.c: unsigned retval makes ctrl_out return 0 in case of error

In my quest to try and figure out why test 14 (control write) doesn't
work with my EZ-USB board, I noticed that sometimes testusb reported
no error even though the kernel log complained "byte 0 is 0 not 2" etc.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>