regmap: Export regmap_async_complete_cb

This fixes below build error when CONFIG_REGMAP=y && CONFIG_REGMAP_SPI=m

ERROR: "regmap_async_complete_cb" [drivers/base/regmap/regmap-spi.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2
Signed-off-by: NAxel Lin <axel.lin@ingics.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/regmap.c

@@ -1615,6 +1615,7 @@ void regmap_async_complete_cb(struct regmap_async *async, int ret)
 	if (wake)
 		wake_up(&map->async_waitq);
 }
+EXPORT_SYMBOL_GPL(regmap_async_complete_cb);
 
 static int regmap_async_is_done(struct regmap *map)
 {