bif: core: set BIF bus state master-disabled when no slaves are found

Modify the bif_ctrl_register() function so that the bus state is
changed to master-disabled if no BIF slaves are detected.  This
can save power when a smart battery pack is not connected.

Change-Id: Ie50203240f10d09656f41e0feab9971bf265e161
Signed-off-by: David Collins <collinsd@codeaurora.org>