haven: dbl: Initialize cap ids to HH_CAPID_INVAL

Currently, the cap-ids for doorbells are initialized to 0. However, 0 is
a valid value for cap-id. Instead, initialize cap-ids to HH_CAPID_INVAL
in the doorbell driver.

Change-Id: Ia1588ed372f0657444e1ab39eab9e6c1e6d3d156
Signed-off-by: Elliot Berman <eberman@codeaurora.org>