twl6030 battery: clear gas gauge at enable time

Otherwise previous register values (from bootloader, or warm reboot)
will distort our calculations.

Change-Id: I302eb3f05cacdc9136c937556dd5a9d5d8baf4d4
Signed-off-by: Sivakumar Pothireddy <sivakumar.pothireddy@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>