twl4030_charger: fix another compile error

When CONFIG_CHARGER_TWL4030=y and CONFIG_TWL4030_MADC=m we get a
compile error:

drivers/built-in.o: In function `twl4030_charger_update_current':
twl4030_charger.c:(.text+0x504681): undefined reference to
`twl4030_get_madc_conversion'

Use IS_REACHABLE to fix it.

Cc: NeilBrown <neil@brown.name>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>