msm: camera: isp: Bound check for number stats registers

The index of used stats register is derived from a stream handle least
significant byte and thus can be up to 255. However the stats registers
are up to 8 depending of the target. Thus a bound check is done before
use of the received stats register index value.

Change-Id: Ic008918f4263f57a5b8aabd34266ac1ba3612a9c
Signed-off-by: Petar Sivenov <psiven@codeaurora.org>