USB: gadget: fix req length in sourcesink_setup()

The recent commit 2edb11cb fixed req->length in the composite_setup()
function, but that will cause all g_zero tests to fail like:

root#> ./testusb -D /proc/bus/usb/002/021 -t14 -c 15000 -s 256 -v 1
unknown speed   /proc/bus/usb/002/021
/proc/bus/usb/002/021 test 14 --> 32 (Broken pipe)

We need to fix req->length in sourcesink_setup() as well to avoid this.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>