regulator: axp20x: Mark expected switch fall-throughs

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

This patch fixes the following warnings:

drivers/regulator/axp20x-regulator.c: In function ‘axp20x_set_dcdc_freq’:
drivers/regulator/axp20x-regulator.c:1023:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   reg = AXP803_DCDC_FREQ_CTRL;
drivers/regulator/axp20x-regulator.c:1025:2: note: here
  case AXP806_ID:
  ^~~~
drivers/regulator/axp20x-regulator.c: In function ‘axp20x_set_dcdc_workmode’:
drivers/regulator/axp20x-regulator.c:1115:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   reg = AXP806_DCDC_MODE_CTRL2;
drivers/regulator/axp20x-regulator.c:1121:2: note: here
  case AXP221_ID:
  ^~~~

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.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>