fix a bug with computation of sqn ("sqn[2] << 24" in an "int" in

C; when implicitely casting to uint64_t the 31st bit is duplicated
to the higher 32 bits; conclusion is easy)


git-svn-id: http://svn.eurecom.fr/openair4G/trunk@7568 818b1a75-f10b-46b9-bf7c-635c3b92a50f