usb: gadget: f_ncm: Fix NTP-32 support

When connecting a CDC-NCM gadget to an host that uses the NTP-32 mode,
or that relies on the default CRC setting, the current implementation gets
confused, and does not expect the correct signature for its packets.

Fix this, by ensuring that the ndp_sign member in the f_ncm structure
always contain a valid value.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>