USB: cdc.h: ncm: add missed constants and structures

Make a dedicated structure for datagram pointer entry. There is no
explicit declaration in the spec, but it's used by the host
implementation and makes the structure more clear.

Add some missed constants from the spec

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>