Changes

gsm48_encode_bearer_cap(): properly set 'Structure' in octet 4

Before commit 714843a45 we hard-coded octet 4 to 0xb8, then
we started hard-coding it to 0x88.  Neither is correct.

According to 3GPP TS 43.010, Annex A.1 "Attribute definition and
their values", attribute 'Structure' can take two values:

* Service data unit integrity, which is
** applicable for connection element "non transparent";
* Unstructured, which is
** applicable for connection element "transparent".

This commit changes the output of tests/gsm0408/gsm0408_test:

* Test `CSD 2400/V.22bis/transparent' is now passing.
* csd_9600_v110_lv[] revert octet 4 back to 0xb8.

Change-Id: I563becfdae75e22de3b5e5de3c810334a74e7ffb
Fixes: 714843a45 ("gsm48_ie.c: change bearer cap structure in outgoing CSD calls")
Related: OS#5730