wm97xx_battery: Clean up some warnings

Staticise work_lock (nothing outside the driver has any reason to
see it) and specify dev when requesting the charger IRQ (since that's
what we pass in when we free it).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>