usb: remove bad dput after dentry_unhash

Commit 64252c75 (vfs: remove dget() from dentry_unhash()) removed the
useless dget from dentry_unhash but didn't fix up this caller in the usb
code.  There used to be exactly one dput per dentry_unhash call; now
there are none.

Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>