wm97xx_battery: Quieten sparse warning (bat_set_pdata not declared)

This patch fixes following sparse warning:

drivers/power/wm97xx_battery.c:311:6: warning: symbol 'wm97xx_bat_set_pdata' was not declared. Should it be static?

Note that we can't just include linux/wm97xx_batt.h because the header
is deprecated, and so this pops up:

In file included from drivers/power/wm97xx_battery.c:22:
include/linux/wm97xx_batt.h:6: warning: #warning This file will be removed soon, use wm97xx.h instead!

Since wm97xx_bat_set_pdata() is also deprecated (in favour of pdata
passed via AC97 bus), just workaround the issue by declaring the
function in wm97xx_battery.c.
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>

drivers/power/wm97xx_battery.c

@@ -308,6 +308,9 @@ static void __exit wm97xx_bat_exit(void)
 	platform_driver_unregister(&wm97xx_bat_driver);
 }
 
+/* The interface is deprecated, as well as linux/wm97xx_batt.h */
+void wm97xx_bat_set_pdata(struct wm97xx_batt_info *data);
+
 void wm97xx_bat_set_pdata(struct wm97xx_batt_info *data)
 {
 	gpdata = data;