Input: pcspkr - 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 just letting the code
fall through to the next case.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/4b659e3e65f2fa3c8bb7ed153e2016c3fb395bbc.1605896059.git.gustavoars@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>