Clear the a device's description string anytime it's driver changes.

Clear the a device's description string anytime it's driver changes.
Descriptions  are specific to drivers and we don't change drivers on attached
devices.  This fixes a few places where we were not clearing the description
when detaching a driver (e.g. with device_attach() failed).  While here, fix
a few other nits:
- Remove spurious call to remove a device's driver from
  devclass_driver_deleted().  device_detach() removes it already.
- Fix a typo.