power: supply: Fix fall-through warnings for Clang

Fix the following fallthrough warnings:

drivers/power/supply/ab8500_fg.c:1730:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/power/supply/abx500_chargalg.c:1155:3: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>