battery: honor the step time in taper stepper algorithm

The taper stepper algorithm doesn't honor 500ms waits. This is because
after parallel's share is changed, the driver goes through the same
enable/disable callback which restarts the algorithm immediately.

To fix it stay in the work until taper stepper exits. The current code
exits the work once the battery enters fast charge and depends on
triggering of charge state change to taper interrupt for further
reduction. Staying the loop precludes the dependency on the interrupt.

Change-Id: I629627d78103caf1bba2ba7381cecd9a0af6d748
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>