regulator: axp20x: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in this particular case, I moved the whole comment
"Fall through to the check below.", which contains the "Fall through"
comment, at the bottom of the case, which is what GCC is expecting
to find.

Addresses-Coverity-ID: 1436594 ("Missing break in switch")
Addresses-Coverity-ID: 1364475 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>