USB: correct locking in gadgetfs_disconnect

This patch moves spin_lock (&dev->lock) before first use of dev.
I think that test to the state of device should be protected with
this spin_lock...

Signed-off-by: Milan Svoboda <msvoboda@ra.rockwell.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>