In osmo-bsc, gsm48_multirate_config() is used in a way where the struct
amr_mode *modes always points at the full set of configured modes, and
only the AMR bits m4_75 thru m12_2 are unset to filter the used modes.
In the current test, the bits are unset to filter, but also the struct
amr_mode *modes is reduced accordingly. Instead, keep the modes fully
populated and unset only bits, like osmo-bsc does in practice.
The test results should not (and do not) differ.
Change-Id: I545de6fb66a4b74a3f08899795b4b4d9c4538f58