Fix USB enumeration on OS X (#640)

Hopefully without breaking the others.

Summary of the changes:
- Device descriptor reports the device is bus powered and requires
  100mA max.
- HID descriptor version bumped to 1.11 (was 1.10)
- Added string index for Interface and HID descriptors (which seems to
  make OS X happy)

boards/nordic/nrf52840dk_opensk/src/main.rs

@@ -119,6 +119,10 @@ static STRINGS: &'static [&'static str] = &[
     "OpenSK",
     // Serial number
     "v1.0",
+    // Interface description + main HID string
+    "FIDO2",
+    // vendor HID string
+    "Vendor HID",
 ];
 
 // State for loading and holding applications.