usb: gadget: ncm: Fix default ndp_sign value

Commit 8d640ad3 ("usb: gadget: ncm: make global variable ndp*_opts
read only") added ndp_sign member to struct f_ncm which is being used
uninitialized when using default NTB format (16 bit with No CRC).
Due to mismatch of ndp_sign value, NCM communication is failing.
Fix this issue by initializing ndp_sign variable with value based
on NTB 16-bit format with No CRC functionality to resolve this issue.
For selecting other NTB format and CRC functionality, Host needs to
use USB_CDC_SET_NTB_FORMAT and USB_CDC_SET_CRC_MODE setup packets and
updating to ndp_sign variable is taken care as part of it.

CRs-Fixed: 798691
Change-Id: I70e88907c18ed2ebe1c0c4c45e1ed32a435ab566
Signed-off-by: Mayank Rana <mrana@codeaurora.org>