a2dp-codec-sbc: get rid of compiler warnings

a2dp-codec-sbc: get rid of compiler warnings

The warnings:

modules/bluetooth/a2dp-codec-sbc.c: In function ‘default_bitpool’:
modules/bluetooth/a2dp-codec-sbc.c:161:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
             switch (mode) {
             ^~~~~~
modules/bluetooth/a2dp-codec-sbc.c:169:9: note: here
         case SBC_SAMPLING_FREQ_44100:
         ^~~~
modules/bluetooth/a2dp-codec-sbc.c:170:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
             switch (mode) {
             ^~~~~~
modules/bluetooth/a2dp-codec-sbc.c:180:9: note: here
         case SBC_SAMPLING_FREQ_48000:
         ^~~~

These were valid warnings in that an invalid channel mode would result
in unintended fallthroughs, but the end result would anyway been a crash
in the pa_assert_not_reached() at the end of the function, so
functionally there's no change.