msm: camera: isp: Improve isp substate logging

Improve sp substate logging to print state string
instead of value. Add handler for reg_update in
bubble state to print a warning log indicate the
event.

CRs-Fixed: 2523062
Change-Id: I95c6f16733a177cbf33099775d8ef7eb049a62b4
Signed-off-by: Pavan Kumar Chilamkurthi <pchilamk@codeaurora.org>