gtp.c: Don't print ""Unknown packet flags" error message in get_seq()

get_seq() is called also from contexts where GTP-U is used, and GTP-U
doesn't have sequence numbers.  Thus, it is perfectly normal if no
sequence number and/or no S flag in the header is present.

Change-Id: Ie19b95bbb4427e547843a019f5213a231a9f83da
1 file changed