Bluetooth: TBS: Fix -Wsometimes-uninitialized warning

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

Signed-off-by: Tom Hughes <tomhughes@chromium.org>