Input: wm97xx - make missing platform data non-fatal

Commit 6480af49

 ("power_supply: wm97xx_battery: use
power_supply_get_drvdata") made wm97xx platform data mandatory, although
it's still optional.

This patch fixes an oops during driver probe on one of my MIPS boards with
a wm9712.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Reviewed-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>