Fix compile warnings with Xcode/clang compiler

Minor changes to UVCCameraControl to get rid of these compile warnings:

```
UVCCameraControl.m:87:17: Using the result of an assignment as a condition without parentheses
UVCCameraControl.m:182:19: Using the result of an assignment as a condition without parentheses
UVCCameraControl.m:284:57: Conflicting parameter types in implementation of 'getRangeForControl:': 'const uvc_control_info_t *' vs 'uvc_control_info_t *'
UVCCameraControl.m:299:51: Conflicting parameter types in implementation of 'getValueForControl:': 'const uvc_control_info_t *' vs 'uvc_control_info_t *'
UVCCameraControl.m:309:64: Conflicting parameter types in implementation of 'setValue:forControl:': 'const uvc_control_info_t *' vs 'uvc_control_info_t *'
```