abis_rsl: do not allow SACCH in MF mode on SAPI=0

osmo-msc does currently not check if an MS attemts to establish an
SACCH link on SAPI=0. This combination is not permitted and should
be dropped.

- Make sure that establish indication for SAPI=0 and channel-type=SACCH
  is not permitted

See also TTCN3 Testcase:
BSC_Tests.TC_rll_est_ind_inval_sacch

Change-Id: I12962072c0ea689d2b9b5c2899ffb59be9310dcb
Closes: OS#2750
1 file changed