battery: wait 500mS between steps for taper stepper

Currently the code only waits 100mS between steps. The hardware
could take longer to deglitch its signals and status. Extend the
wait to 500mS.

Change-Id: I4a037f85f43bc670c5a357e4ecb46962b0d33598
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>