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>