msm: camera: common: Fix cpas register for custom hw

The client handle is obtained by providing client identifier
to cpas. Add identifier in custom hw registration for cpas
to recognize client. Also, improve identifier string
validation in cpas register client.

CRs-Fixed: 2528679
Change-Id: I924f5c98a8affa26753710fa542c1bb651d0af13
Signed-off-by: Mukund Madhusudan Atre <matre@codeaurora.org>