This fixes a problem that showed in usb-storage (osdl bugme 1310)
and could have shown in other drivers that used usb_reset_device()
when they already held dev->serialize:  a self-deadlock with some
devices.

There are some drivers that should likely change so that they grab
this lock themselves, since they don't call this during probe()
when the lock is already held.  The lock protects against config
changes by other tasks, which is currently quite rate.