stp: add and use f_m3ua_{cli,srv}_config() helpers

Using this API reduces the risk of using the wrong half of the
mp_m3ua_configs[] array and improves the code readability a bit.

Change-Id: I9788f52c20574f4f9d015d2de11b5e42bb03d15f
Related: SYS#5424
1 file changed