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>

drivers/power/twl4030_charger.c

@@ -91,7 +91,7 @@
 #define TWL4030_MSTATEC_COMPLETE1	0x0b
 #define TWL4030_MSTATEC_COMPLETE4	0x0e
 
-#if IS_ENABLED(CONFIG_TWL4030_MADC)
+#if IS_REACHABLE(CONFIG_TWL4030_MADC)
 /*
  * If AC (Accessory Charger) voltage exceeds 4.5V (MADC 11)
  * then AC is available.