pmic-voter: remove default value

The default value was introduced to prevent crashes on the clients
where it couldn't handle situations when there were no voters.
The default value led to many complicated scenarios in the votable
implementation.

Remove the concept of default value and return error when
there are no votes. Update all the clients to handle error values
from votables. Also invoke the callback to inform about no active
voters.

CRs-Fixed: 1018090
Change-Id: I81f5974f81a697f3698bc58df1d3ed59fa2579a7
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>