usb: gadget: composite: fix req->length in composite_setup()

When USB CV MSC tests are run on f_mass_storage gadget
Bulk Only Mass Storage Reset fails since req->length
is set to USB_BUFSIZ=1024 in composite_setup().

Initialize req->length to zero to fix this.

Signed-off-by: Maulik Mankad <maulik@ti.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>