msm: camera: cpas: Avoid array underflow during client registration

CPAS driver maintains an array of clients and accordingly takes the
lock on them. While registering the client if the client name is not
present in the existing clients' list, CPAS driver should avoid the
registration and throw error.
This change introduces the same and hence avoiding array underflow.

Change-Id: I537b99b0b1e76afa60834f546dfc2e622b203dac
Signed-off-by: Rishabh Jain <risjai@codeaurora.org>