os/darwin: use IOUSBDevice as darwin_device_class explicitly

os/darwin: use IOUSBDevice as darwin_device_class explicitly

kIOUSBDeviceClassName define from IOUSBLib.h file was changed from
IOUSBDevice to IOUSBHostDevice in macOS Catalina.

In previous macOS versions, it was always defined as IOUSBDevice:
```
 #define kIOUSBDeviceClassName		 "IOUSBDevice"
```
In macOS Catalina it looks as follows:
```
 #if MAC_OS_X_VERSION_MIN_REQUIRED <= MAC_OS_X_VERSION_10_14
 #define kIOUSBDeviceClassName           "IOUSBDevice"
 #define kIOUSBInterfaceClassName        "IOUSBInterface"
 #else
 #define kIOUSBDeviceClassName           kIOUSBHostDeviceClassName
 #define kIOUSBInterfaceClassName        kIOUSBHostInterfaceClassName
 #endif /* MAC_OS_X_VERSION_MIN_REQUIRED <= MAC_OS_X_VERSION_10_14 */
```

By default, macOS Catalina uses IOUSBHostDevice. The problem is that
using the IOUSBHostDevice class misses some devices.  This has been
described in 044a7ee commit. To work only with IOUSBDevice,
darwin_device_class has been explicitly set to IOUSBDevice.

Closes #693

For reference, here's the output of the 'listdevs' example before my changes:
05ac:027b (bus 128, device 2) path: 5
2109:0102 (bus 20, device 6) path: 4
05e3:0751 (bus 20, device 5) path: 1
1a40:0801 (bus 20, device 3) path: 2
0bda:8153 (bus 0, device 2) path: 3
2109:2817 (bus 20, device 1) path: 8
2109:0817 (bus 0, device 1) path: 4

And here's the output after my changes:
05ac:027b (bus 128, device 2) path: 5
2109:0102 (bus 20, device 6) path: 3.2.4
05e3:0751 (bus 20, device 5) path: 3.2.1
1a40:0801 (bus 20, device 3) path: 3.2
0bda:8153 (bus 0, device 2) path: 2.3
2109:2817 (bus 20, device 1) path: 3
2109:0817 (bus 0, device 1) path: 2

Closes #712

Signed-off-by: Nathan Hjelm <hjelmn@google.com>