ALSA: sb: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/0f6d5b94f1890be2b8f88d6f930fc75779e26345.1605896059.git.gustavoars@kernel.org


Signed-off-by: Takashi Iwai <tiwai@suse.de>