power: supply: ab8500_charger: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: allmodconfig arm):

drivers/power/supply/ab8500_charger.c:738:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>