libusbhost: Skip /dev inotify if /dev/bus/usb already exists

Skip watching /dev changes if the final /dev/bus/usb already
exists.  Register only for /dev/bus in this scenario on systems
where the last "/usb" path may be deleted (currently handled by
libusbhost).

Removal of inotify watch on /dev reduces runtime overhead as
/dev nodes are modified frequently.

Change-Id: I6823a68c3977ae01832808ae5f8c32fa913b02cf