regulator: as3722: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword 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/c0efb81064f71837f19408f65b52d155103ee514.1605896059.git.gustavoars@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>