libmsc/gsm_04_11.c: refactor RP-DATA header validation

It's much better to have both RP-DATA header parsing and validation
code in a single function. There is no need to pass all the header
fields (DA, OA, UI) to gsm411_rx_rp_ud() because they are not
used there.

Change-Id: Iaf295949148e2a613c5403d1f7a926fcd6849c15
1 file changed