msm: camera: Return correct code when camera open fails

There can be situations when opening camera will fail, not
due to an error but due to the fact that the current hardware
configuration can only support either the front or back camera
and can't support opening another the camera at the moment.

In such cases, rather than returning -EFAULT to user space, camera
should return -EBUSY so as to distinguish between a genuine failure
to open the camera and a failure due to current configuration which
precludes from opening another instance of the camera.

So, correct the return code in the camera open failure handling code.

Change-Id: Ib99ed919be44b40a1b0270a3d2a8a1794a0373cb
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>