drivers: usb: Fix config indentation

The indentation was messing up with the scripts/build-whitelist.sh that
was marking SYS_USB_EVENT_POLL_VIA_INT_QUEUE (and probably also the
other indented options) erroneously as ad-hoc configure option with the
following error:

```
Error: You must add new CONFIG options using Kconfig
The following new ad-hoc CONFIG options were detected:
CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE
```