Fix USB drive scanning with multiple drives (WS-269)

Buggy logic from FlexBridgeBeta added in 78439011 was causing only the
first drive to be considered. This doesn't work well in Balsa where
the root drive is typically a USB drive too.
Merge pull request #31 from neilmayhew/topic/usb-enumeration

Fix USB drive scanning with multiple drives (WS-269)