cdc_ncm: Fix the build warning

The ndp32->wLength is two bytes long, so replace cpu_to_le32 with cpu_to_le16.

Fixes: 0fa81b30 ("cdc_ncm: Implement the 32-bit version of NCM Transfer Block")
Signed-off-by: Alexander Bersenev <bay@hackerdom.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>