msm: camera: Add support to return session id through g_ctrl ioctl

User space needs a mechanism to return the current session id. Use the
g_ctrl ioctl to return the session id.

Change-Id: I8404c8d627c771e0a2f4535d6f66b1b85288808a
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>