wm97xx_battery: Use DEFINE_MUTEX() for work_lock

work_lock can be initialized automatically with
DEFINE_MUTEX() rather than explicitly calling mutex_init().

This patch also removes an unused bat_lock mutex.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>