Bluetooth: TBS: Fix -Wsometimes-uninitialized warning

[thughes]

Building bluetooth.shell.audio.only_gtbs with clang warns:

subsys/bluetooth/audio/tbs.c:1646:6: error: variable 'ret' is used uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]
        if (param->gtbs) {
            ^~~~~~~~~~~
subsys/bluetooth/audio/tbs.c:1667:9: note: uninitialized use occurs here
        return ret;
               ^~~
subsys/bluetooth/audio/tbs.c:1646:2: note: remove the 'if' if its condition is always true
        if (param->gtbs) {
        ^~~~~~~~~~~~~~~~~
subsys/bluetooth/audio/tbs.c:1626:9: note: initialize the variable 'ret' to silence this warning
        int ret;
               ^
                = 0

[Thalley]

This is fine as it fixes an issue.

It can only happen if CHECKIF is not enabled, otherwise it would always go into one of the branches, so perhaps we could also add an __ASSERT_NO_MSG(ret != -ENOEXEC) before the return ret?